feat(sidebar): resizable width with drag handle + persistence (t-paliad-047)

Adds a 6px col-resize strip on the right edge of the desktop sidebar.
Drag updates --sidebar-width on document.documentElement (clamped
180-480px). Mouse + touch handlers; double-click resets to the 240px
default. Width persists via localStorage["paliad-sidebar-width"], read
on every page load before first paint so layout is stable from frame 1.

The handle is opacity-faded on the icon-rail (collapsed) state and
hidden entirely under the mobile breakpoint, since the mobile sidebar
is an overlay drawer that always uses the fixed --sidebar-expanded
width.

Pin/unpin behaviour is preserved: pinned state keeps the user's chosen
width; unpinning drops to the icon rail; hover-expand restores the
chosen width. The hover-collapse mouseleave handler ignores transitions
during an active drag so the sidebar doesn't snap shut mid-resize.

frontend/src/client/i18n.ts

@@ -792,6 +792,8 @@ const translations: Record<Lang, Record<string, string>> = {
     "palette.footer.open": "Öffnen",
     "palette.footer.close": "Schließen",
 
+    "sidebar.resize.title": "Breite anpassen — ziehen, Doppelklick setzt zurück",
+
     // Settings page (t-paliad-022)
     "einstellungen.title": "Einstellungen \u2014 Paliad",
     "einstellungen.heading": "Einstellungen",
@@ -1953,6 +1955,8 @@ const translations: Record<Lang, Record<string, string>> = {
     "palette.footer.open": "Open",
     "palette.footer.close": "Close",
 
+    "sidebar.resize.title": "Resize \u2014 drag, double-click to reset",
+
     // Settings page (t-paliad-022)
     "einstellungen.title": "Settings \u2014 Paliad",
     "einstellungen.heading": "Settings",

frontend/src/client/sidebar.ts

@@ -5,6 +5,10 @@ import { initGlobalSearch } from "./search";
 import { getChangelogSeen } from "./changelog-seen";
 const PIN_KEY = "paliad-sidebar-pinned";
 const LEGACY_PIN_KEY = "patholo-sidebar-pinned";
+const WIDTH_KEY = "paliad-sidebar-width";
+const SIDEBAR_WIDTH_MIN = 180;
+const SIDEBAR_WIDTH_MAX = 480;
+const SIDEBAR_WIDTH_DEFAULT = 240;
 
 // toggleMobileSidebar opens or closes the slide-out drawer. Exposed so the
 // BottomNav menu slot can call it without duplicating the open/close
@@ -25,6 +29,24 @@ export function toggleMobileSidebar(): void {
   }
 }
 
+// readStoredWidth returns the user's persisted sidebar width in px, falling
+// back to SIDEBAR_WIDTH_DEFAULT when missing, malformed, or outside the
+// supported clamp range. Stored values are written by initSidebarResize on
+// dragend; the read happens on every page load (initial CSS-var apply).
+function readStoredWidth(): number {
+  const raw = localStorage.getItem(WIDTH_KEY);
+  if (raw === null) return SIDEBAR_WIDTH_DEFAULT;
+  const n = parseInt(raw, 10);
+  if (!Number.isFinite(n) || n < SIDEBAR_WIDTH_MIN || n > SIDEBAR_WIDTH_MAX) {
+    return SIDEBAR_WIDTH_DEFAULT;
+  }
+  return n;
+}
+
+function applySidebarWidth(px: number): void {
+  document.documentElement.style.setProperty("--sidebar-width", `${px}px`);
+}
+
 // migrateLegacyPinKey copies the pre-rebrand pin state into the new key on
 // first load and removes the stale entry. Drop this fallback once the rename
 // grace period is over.
@@ -39,11 +61,16 @@ function migrateLegacyPinKey(): void {
 
 export function initSidebar() {
   migrateLegacyPinKey();
+  // Apply the persisted width before any sidebar paint so the layout is
+  // stable from frame 1. Safe to call before the .sidebar element exists —
+  // the CSS var lives on document.documentElement.
+  applySidebarWidth(readStoredWidth());
   initInviteModal();
   initGlobalSearch();
   initChangelogBadge();
   const sidebar = document.querySelector<HTMLElement>(".sidebar");
   if (!sidebar) return;
+  initSidebarResize(sidebar);
 
   const pinBtn = sidebar.querySelector<HTMLButtonElement>(".sidebar-pin");
   const hamburger = document.querySelector<HTMLButtonElement>(".sidebar-hamburger");
@@ -74,6 +101,10 @@ export function initSidebar() {
       clearTimeout(hoverTimer);
       hoverTimer = null;
     }
+    // Mid-resize drags briefly pull the cursor off the sidebar — keep
+    // the sidebar expanded so the user can finish the drag. The resizing
+    // class is cleared on dragend; the next mouseleave will collapse.
+    if (sidebar.classList.contains("resizing")) return;
     sidebar.classList.remove("expanded");
   });
 
@@ -145,6 +176,99 @@ export function initSidebar() {
   });
 }
 
+// initSidebarResize wires the right-edge drag handle: mousedown/touchstart
+// captures the starting cursor position and current width, mousemove/touchmove
+// applies the delta as --sidebar-width (clamped), mouseup/touchend persists
+// the final value to localStorage. Double-click on the handle resets to the
+// default width. The .resizing class on the sidebar suppresses both the
+// width transition (avoid jitter while dragging) and the hover-collapse
+// (the cursor leaves the sidebar mid-drag).
+function initSidebarResize(sidebar: HTMLElement): void {
+  const handle = sidebar.querySelector<HTMLElement>(".sidebar-resize-handle");
+  if (!handle) return;
+
+  let dragging = false;
+  let startX = 0;
+  let startWidth = 0;
+
+  function applyDelta(clientX: number): void {
+    const delta = clientX - startX;
+    let next = startWidth + delta;
+    if (next < SIDEBAR_WIDTH_MIN) next = SIDEBAR_WIDTH_MIN;
+    if (next > SIDEBAR_WIDTH_MAX) next = SIDEBAR_WIDTH_MAX;
+    applySidebarWidth(next);
+  }
+
+  function readCurrentWidth(): number {
+    const raw = getComputedStyle(document.documentElement)
+      .getPropertyValue("--sidebar-width")
+      .trim();
+    const n = parseInt(raw, 10);
+    return Number.isFinite(n) ? n : SIDEBAR_WIDTH_DEFAULT;
+  }
+
+  function endDrag(): void {
+    if (!dragging) return;
+    dragging = false;
+    sidebar.classList.remove("resizing");
+    document.body.classList.remove("sidebar-resizing");
+    document.removeEventListener("mousemove", onMouseMove);
+    document.removeEventListener("mouseup", endDrag);
+    document.removeEventListener("touchmove", onTouchMove);
+    document.removeEventListener("touchend", endDrag);
+    document.removeEventListener("touchcancel", endDrag);
+    localStorage.setItem(WIDTH_KEY, String(readCurrentWidth()));
+  }
+
+  function onMouseMove(e: MouseEvent): void {
+    applyDelta(e.clientX);
+  }
+
+  function onTouchMove(e: TouchEvent): void {
+    if (e.touches.length === 0) return;
+    applyDelta(e.touches[0].clientX);
+    // preventDefault stops the browser from interpreting the drag as a
+    // page scroll. touch-action: none on the handle covers most browsers,
+    // but Safari still needs the explicit call.
+    if (e.cancelable) e.preventDefault();
+  }
+
+  function startDrag(clientX: number): void {
+    dragging = true;
+    startX = clientX;
+    // Use the rendered sidebar width — robust against any future state
+    // (pinned vs hover-expanded) where the CSS var might not be the
+    // single source of truth.
+    startWidth = sidebar.getBoundingClientRect().width;
+    sidebar.classList.add("resizing");
+    document.body.classList.add("sidebar-resizing");
+  }
+
+  handle.addEventListener("mousedown", (e) => {
+    // Only primary button. Right/middle clicks would otherwise stick the
+    // sidebar in resizing state until a real mouseup arrives.
+    if (e.button !== 0) return;
+    e.preventDefault();
+    startDrag(e.clientX);
+    document.addEventListener("mousemove", onMouseMove);
+    document.addEventListener("mouseup", endDrag);
+  });
+
+  handle.addEventListener("touchstart", (e) => {
+    if (e.touches.length === 0) return;
+    startDrag(e.touches[0].clientX);
+    document.addEventListener("touchmove", onTouchMove, { passive: false });
+    document.addEventListener("touchend", endDrag);
+    document.addEventListener("touchcancel", endDrag);
+  }, { passive: true });
+
+  handle.addEventListener("dblclick", (e) => {
+    e.preventDefault();
+    applySidebarWidth(SIDEBAR_WIDTH_DEFAULT);
+    localStorage.setItem(WIDTH_KEY, String(SIDEBAR_WIDTH_DEFAULT));
+  });
+}
+
 // Changelog badge — fetches the count of entries newer than the locally
 // stored "last seen" stamp and renders a dot + number on the Neuigkeiten
 // link. Skipped on the changelog page itself because changelog.ts stamps

frontend/src/components/Sidebar.tsx

@@ -161,6 +161,13 @@ export function Sidebar({ currentPath, authenticated = true }: SidebarProps): st
             <span className="sidebar-label" data-i18n="nav.logout">Abmelden</span>
           </a>
         </div>
+
+        {/* Drag-strip on the right edge — sidebar.ts wires mouse/touch
+            handlers and persists the chosen width to localStorage.
+            CSS hides it when the sidebar is collapsed/icon-rail or on mobile. */}
+        <div className="sidebar-resize-handle" role="separator" aria-orientation="vertical"
+             aria-label="Resize sidebar"
+             title="Breite anpassen" data-i18n-title="sidebar.resize.title" />
       </aside>
 
       <button className="sidebar-hamburger" type="button" aria-label="Menu">

frontend/src/styles/global.css

@@ -18,6 +18,12 @@
     --max-width: 1080px;
     --sidebar-collapsed: 64px;
     --sidebar-expanded: 240px;
+    /* User-adjustable width for the expanded/pinned desktop sidebar.
+       Defaults to --sidebar-expanded; sidebar.ts overrides on
+       document.documentElement when the user drags the resize handle
+       and persists the value to localStorage. Mobile drawer keeps the
+       fixed --sidebar-expanded width. */
+    --sidebar-width: 240px;
     --bottom-nav-height: 56px;
 }
 
@@ -493,7 +499,13 @@ main {
 
 .sidebar.expanded,
 .sidebar.pinned {
-    width: var(--sidebar-expanded);
+    width: var(--sidebar-width);
+}
+
+.sidebar.resizing,
+.sidebar.resizing .sidebar-label,
+.has-sidebar.sidebar-resizing {
+    transition: none !important;
 }
 
 .sidebar-header {
@@ -556,6 +568,38 @@ main {
     color: var(--color-accent);
 }
 
+/* Vertical drag-strip on the sidebar's right edge. Only meaningful when
+   the sidebar is wide enough to show labels (expanded or pinned), so
+   collapsed/icon-rail state hides it. Positioned slightly off-edge for
+   easier hit; pointer-events stay enabled only when visible. */
+.sidebar-resize-handle {
+    position: absolute;
+    top: 0;
+    right: -3px;
+    width: 6px;
+    height: 100%;
+    cursor: col-resize;
+    z-index: 41;
+    background: transparent;
+    opacity: 0;
+    pointer-events: none;
+    transition: opacity 150ms ease, background 150ms ease;
+    user-select: none;
+    touch-action: none;
+}
+
+.sidebar.expanded .sidebar-resize-handle,
+.sidebar.pinned .sidebar-resize-handle {
+    opacity: 1;
+    pointer-events: auto;
+}
+
+.sidebar-resize-handle:hover,
+.sidebar.resizing .sidebar-resize-handle {
+    background: var(--color-accent);
+    opacity: 0.5;
+}
+
 .sidebar-nav {
     display: flex;
     flex-direction: column;
@@ -697,7 +741,7 @@ main {
 }
 
 .has-sidebar.sidebar-pinned {
-    padding-left: var(--sidebar-expanded);
+    padding-left: var(--sidebar-width);
 }
 
 .has-sidebar .tool-results {
@@ -1565,6 +1609,10 @@ input[type="range"]::-moz-range-thumb {
         display: none;
     }
 
+    .sidebar-resize-handle {
+        display: none;
+    }
+
     .sidebar-hamburger {
         display: flex;
     }
@@ -5686,7 +5734,7 @@ input[type="range"]::-moz-range-thumb {
     position: fixed;
     top: 6.75rem;
     left: 0.5rem;
-    width: calc(var(--sidebar-expanded) - 1rem);
+    width: calc(var(--sidebar-width) - 1rem);
     max-width: 420px;
     max-height: calc(100vh - 8rem);
     overflow-y: auto;