feat(projects-detail): "Untergeordnet" tab → "Projektbaum" with full visible hierarchy

m typed in another pane: "The project view where there is a tab
'Untergeordnet' I want a 'Project Tree' instead. And it always shows
all siblings, all parents and all children of that entity." (Forwarded
by klaus / youpcorg/head, msg #1570.)

Tab label
  DE: Untergeordnet → Projektbaum
  EN: Sub-projects → Project Tree
  i18n key kept as projects.detail.tab.kinder for back-compat (legacy
  bookmarks + create-sub-project CTA still keyed on 'kinder').

Tree content
  Was: direct children only (one /api/projects/<id>/children call).
  Now: full visible project hierarchy via /api/projects/tree?subtree_counts=false,
  rendered as nested <ul> with the current node highlighted with a
  lime-soft background + current-color border. The dashed left border
  on nested levels makes parent → child relationships scannable.
  Visibility is RLS-scoped (the tree endpoint already filters to projects
  the user can see).

Empty state
  "Keine untergeordneten Projekte" still renders when the current node
  has zero direct children — that is what the "+ Untervorhaben anlegen"
  CTA next to it actually creates. Showing it for "tree has no other
  branches" would have been wrong.

The standalone /api/projects/<id>/children call stays — it gates the
empty state and pre-fills parent_id on the create form.

frontend/src/client/i18n.ts

@@ -1095,7 +1095,7 @@ const translations: Record<Lang, Record<string, string>> = {
     "projects.detail.save": "Speichern",
     "projects.detail.tab.verlauf": "Verlauf",
     "projects.detail.tab.team": "Team",
-    "projects.detail.tab.kinder": "Untergeordnet",
+    "projects.detail.tab.kinder": "Projektbaum",
     "projects.detail.tab.parteien": "Parteien",
     "projects.detail.tab.fristen": "Fristen",
     "projects.detail.tab.termine": "Termine",
@@ -3166,7 +3166,7 @@ const translations: Record<Lang, Record<string, string>> = {
     "projects.detail.save": "Save",
     "projects.detail.tab.verlauf": "Activity",
     "projects.detail.tab.team": "Team",
-    "projects.detail.tab.kinder": "Sub-projects",
+    "projects.detail.tab.kinder": "Project Tree",
     "projects.detail.tab.parteien": "Parties",
     "projects.detail.tab.fristen": "Deadlines",
     "projects.detail.tab.termine": "Appointments",

frontend/src/client/projects-detail.ts

@@ -206,6 +206,8 @@ let deadlines: Deadline[] = [];
 let appointments: Appointment[] = [];
 let ancestors: ProjectMini[] = [];
 let children: ProjectMini[] = [];
+type TreeNode = ProjectMini & { children?: TreeNode[] };
+let projectTree: TreeNode[] = [];
 let teamMembers: ProjectTeamMember[] = [];
 // t-paliad-139 — additional Team-tab sections.
 let descendantStaffed: ProjectTeamMember[] = [];
@@ -1534,38 +1536,63 @@ function renderBreadcrumb() {
   el.innerHTML = crumbs.join(BREADCRUMB_CHEVRON);
 }
 
-// ----- Children -----------------------------------------------------------
+// ----- Project Tree (Projektbaum) -----------------------------------------
+// Renders the full visible project hierarchy with the current node highlighted.
+// One round-trip to /api/projects/tree gets every project the user can see;
+// the renderer walks the tree and produces a nested <ul> with the current
+// node visually marked. Direct children of the current node still drive the
+// "no sub-projects" empty state, since that is the actionable signal for
+// the "+ Untervorhaben anlegen" CTA.
 
 async function loadChildren(id: string) {
+  // Direct children kept for the "Keine untergeordneten Projekte" empty state
+  // and the create-new pre-fill (parent_id from the current node).
   try {
     const resp = await fetch(`/api/projects/${id}/children`);
     if (resp.ok) children = ((await resp.json()) as ProjectMini[]) ?? [];
   } catch {
     children = [];
   }
+  // Full visible tree drives the rendering.
+  try {
+    const resp = await fetch(`/api/projects/tree?subtree_counts=false`);
+    if (resp.ok) projectTree = ((await resp.json()) as TreeNode[]) ?? [];
+  } catch {
+    projectTree = [];
+  }
 }
 
 function renderChildren() {
-  const list = document.getElementById("children-list")!;
+  const root = document.getElementById("project-tree")!;
   const empty = document.getElementById("children-empty")!;
-  if (!children.length) {
-    list.innerHTML = "";
-    empty.style.display = "";
+  // Empty state only when the current node has zero direct children — the
+  // CTA next to the empty message is "create sub-project", which would be
+  // misleading if the tree itself has other branches.
+  empty.style.display = children.length ? "none" : "";
+  if (!projectTree.length) {
+    root.innerHTML = "";
     return;
   }
-  empty.style.display = "none";
-  list.innerHTML = children
-    .map(
-      (c) => `<li class="projekt-child-item">
-        <a href="/projects/${esc(c.id)}" class="projekt-child-link">
-          <span class="entity-type-chip entity-type-${esc(c.type)}">${esc(tDyn("projects.type." + c.type) || c.type)}</span>
-          <span class="projekt-child-title">${esc(c.title)}</span>
-          ${c.reference ? `<span class="projekt-child-ref">${esc(c.reference)}</span>` : ""}
-          <span class="entity-status-chip entity-status-${esc(c.status)}">${esc(tDyn("projects.filter.status." + c.status) || c.status)}</span>
-        </a>
-      </li>`,
-    )
-    .join("");
+  const currentId = project?.id ?? "";
+  root.innerHTML = `<ul class="projekt-tree-list">${projectTree.map((n) => renderTreeNode(n, currentId, 0)).join("")}</ul>`;
+}
+
+function renderTreeNode(node: TreeNode, currentId: string, depth: number): string {
+  const isCurrent = node.id === currentId;
+  const kids = node.children ?? [];
+  const childHTML = kids.length
+    ? `<ul class="projekt-tree-list">${kids.map((k) => renderTreeNode(k, currentId, depth + 1)).join("")}</ul>`
+    : "";
+  const itemClass = `projekt-tree-item${isCurrent ? " projekt-tree-item--current" : ""}`;
+  const linkInner =
+    `<span class="entity-type-chip entity-type-${esc(node.type)}">${esc(tDyn("projects.type." + node.type) || node.type)}</span>` +
+    `<span class="projekt-tree-title">${esc(node.title)}</span>` +
+    (node.reference ? `<span class="projekt-tree-ref">${esc(node.reference)}</span>` : "") +
+    `<span class="entity-status-chip entity-status-${esc(node.status)}">${esc(tDyn("projects.filter.status." + node.status) || node.status)}</span>`;
+  const row = isCurrent
+    ? `<span class="projekt-tree-link projekt-tree-link--current" aria-current="true">${linkInner}</span>`
+    : `<a href="/projects/${esc(node.id)}" class="projekt-tree-link">${linkInner}</a>`;
+  return `<li class="${itemClass}">${row}${childHTML}</li>`;
 }
 
 function initChildAddLink() {

frontend/src/projects-detail.tsx

@@ -73,7 +73,7 @@ export function renderProjectsDetail(): string {
                 <nav className="entity-tabs" id="project-tabs">
                   <a className="entity-tab" data-tab="history" href="#" data-i18n="projects.detail.tab.verlauf">Verlauf</a>
                   <a className="entity-tab" data-tab="team" href="#" data-i18n="projects.detail.tab.team">Team</a>
-                  <a className="entity-tab" data-tab="children" href="#" data-i18n="projects.detail.tab.kinder">Untergeordnet</a>
+                  <a className="entity-tab" data-tab="children" href="#" data-i18n="projects.detail.tab.kinder">Projektbaum</a>
                   <a className="entity-tab" data-tab="parties" href="#" data-i18n="projects.detail.tab.parteien">Parteien</a>
                   <a className="entity-tab" data-tab="deadlines" href="#" data-i18n="projects.detail.tab.fristen">Fristen</a>
                   <a className="entity-tab" data-tab="appointments" href="#" data-i18n="projects.detail.tab.termine">Termine</a>
@@ -250,14 +250,14 @@ export function renderProjectsDetail(): string {
                   </div>
                 </section>
 
-                {/* Children (Untergeordnet) */}
+                {/* Project Tree (Projektbaum) — full visible hierarchy with current node highlighted */}
                 <section className="entity-tab-panel" id="tab-children" style="display:none">
                   <div className="party-controls">
                     <a id="child-add-link" className="btn-primary btn-cta-lime btn-small" href="/projects/new" data-i18n="projects.detail.kinder.add">
                       Untervorhaben anlegen
                     </a>
                   </div>
-                  <ul id="children-list" className="projekt-children-list" />
+                  <div id="project-tree" className="projekt-tree" />
                   <p className="entity-events-empty" id="children-empty" style="display:none" data-i18n="projects.detail.kinder.empty">
                     Keine untergeordneten Projekte.
                   </p>

frontend/src/styles/global.css

@@ -9307,6 +9307,64 @@ dialog.quick-add-sheet::backdrop {
     opacity: 0.55;
 }
 
+/* Projektbaum / Project Tree — full visible hierarchy on /projects/<id>. */
+.projekt-tree {
+    margin-top: 0.5rem;
+}
+
+.projekt-tree-list {
+    list-style: none;
+    padding-left: 0;
+    margin: 0;
+}
+
+.projekt-tree-list .projekt-tree-list {
+    padding-left: 1.5rem;
+    border-left: 1px dashed var(--color-border-subtle, #e4e4e7);
+    margin-left: 0.6rem;
+}
+
+.projekt-tree-item {
+    margin: 0.15rem 0;
+}
+
+.projekt-tree-link {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.5rem;
+    padding: 0.25rem 0.5rem;
+    border-radius: 6px;
+    text-decoration: none;
+    color: var(--color-text, var(--hlc-midnight));
+    line-height: 1.3;
+}
+
+.projekt-tree-link:hover,
+.projekt-tree-link:focus-visible {
+    background: var(--color-bg-subtle);
+}
+
+.projekt-tree-link--current {
+    background: var(--color-cta-lime-soft, #f0fad9);
+    border: 1px solid var(--color-cta-lime, #c6f41c);
+    font-weight: 600;
+    cursor: default;
+}
+
+.projekt-tree-link--current:hover {
+    background: var(--color-cta-lime-soft, #f0fad9);
+}
+
+.projekt-tree-title {
+    font-weight: 500;
+}
+
+.projekt-tree-ref {
+    font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
+    font-size: 0.85em;
+    color: var(--color-text-muted);
+}
+
 @media (max-width: 640px) {
     .projekt-breadcrumb {
         flex-wrap: nowrap;