feat(procedures): U1 fold Mode A (Direkt suchen) (m/paliad#151)

Mounts mountModeA() into #procedures-panel-search when the Direkt-suchen
tab activates. The legacy fristenrechner-mode-a code runs unchanged
inside a wrapper that reseeds the #fristen-overhaul-root /
#fristen-overhaul-mode-host scaffold on every tab activation, so
re-clicking the tab always restores a fresh Mode A surface even if the
previous interaction committed an event into the result view.

`?event=<code>` deep links still resolve: boot detects the param,
activates the search tab, and hands directly to mountResultView() —
the result lands inside the same root, the user sees the picked
event's follow-up rules with the Direkt-suchen tab as the visible
context.

Search-box-in-filter-strip composition with chip filters (m's Q3
divergence) lands later, after Mode B + Verfahrensablauf are folded —
the unified state machine pulls all three behind one search input.

frontend/src/client/procedures.ts

@@ -1,23 +1,20 @@
-// /tools/procedures client — U0 skeleton boot
-// (m/paliad#151, docs/design-unified-procedural-events-tool-2026-05-27.md).
+// /tools/procedures client (m/paliad#151,
+// docs/design-unified-procedural-events-tool-2026-05-27.md).
 //
-// U0 owns:
-//   - Tab switching across the four entry modes (proceeding / search /
-//     wizard / akte). URL-driven via ?mode=<name>; cold open lands on
-//     "proceeding" per design §11.5.Q3.
-//   - Showing exactly one panel at a time.
+// Boot logic + tab switching for the unified procedural-events tool.
+// Each entry tab mounts its own module; the search box and chip
+// filters in the top filter strip are wired in U1+ as each slice adds
+// its dimension-aware behaviour.
 //
-// Later slices wire data:
-//   U1 mounts Mode A into #procedures-panel-search.
-//   U2 mounts Mode B into #procedures-panel-wizard.
-//   U3 mounts Verfahrensablauf into #procedures-panel-proceeding +
-//      #procedures-output-tree.
-//   The search box in the top filter strip and the chip filters in
-//   #procedures-filter-chips-* are also wired in U1+ as each slice
-//   adds its dimension-aware behaviour.
+// U0 — Skeleton + tab toggling.
+// U1 — Direkt suchen mounts Mode A (this slice).
+// U2 — Geführt mounts Mode B wizard.
+// U3 — Verfahren wählen mounts Verfahrensablauf tree + 3-way detail filter.
 
 import { initI18n } from "./i18n";
 import { initSidebar } from "./sidebar";
+import { mountModeA } from "./fristenrechner-mode-a";
+import { mountResultView } from "./fristenrechner-result";
 
 type ProceduresTab = "proceeding" | "search" | "wizard" | "akte";
 
@@ -40,7 +37,24 @@ function writeTabToUrl(tab: ProceduresTab): void {
   history.replaceState(null, "", url.pathname + url.search + url.hash);
 }
 
-function showTab(tab: ProceduresTab): void {
+// ensureSearchHost re-seeds the overhaul-root / mode-host scaffold
+// inside the Direkt-suchen panel. Idempotent — re-clicking the search
+// tab always restores a fresh Mode A surface, even if the user
+// previously committed an event into the result view (which had
+// replaced the root's innerHTML).
+function ensureSearchHost(): void {
+  const panel = document.getElementById("procedures-panel-search");
+  if (!panel) return;
+  panel.innerHTML = `
+    <div class="procedures-mode-a-host">
+      <div class="fristen-overhaul-root" id="fristen-overhaul-root">
+        <div id="fristen-overhaul-mode-host"></div>
+      </div>
+    </div>
+  `;
+}
+
+function setActiveTabUI(tab: ProceduresTab): void {
   for (const t of TABS) {
     const btn = document.getElementById(`procedures-tab-${t}`);
     const panel = document.getElementById(`procedures-panel-${t}`);
@@ -53,20 +67,52 @@ function showTab(tab: ProceduresTab): void {
   }
 }
 
+async function activateTab(tab: ProceduresTab): Promise<void> {
+  setActiveTabUI(tab);
+  if (tab === "search") {
+    ensureSearchHost();
+    await mountModeA();
+  }
+  // U2 will mount the wizard here; U3 will mount Verfahrensablauf.
+}
+
 function wireTabs(): void {
   for (const t of TABS) {
     const btn = document.getElementById(`procedures-tab-${t}`);
     if (!btn) continue;
     btn.addEventListener("click", () => {
-      showTab(t);
+      void activateTab(t);
       writeTabToUrl(t);
     });
   }
 }
 
+// boot dispatches on the URL: a deep link with `?event=` jumps straight
+// to the linear result view (the Direkt-suchen tab stays as the visible
+// context). Otherwise the requested tab — defaulting to "proceeding" —
+// activates per readTabFromUrl().
+async function boot(): Promise<void> {
+  const params = new URLSearchParams(window.location.search);
+  const eventRef = params.get("event") || "";
+
+  if (eventRef) {
+    setActiveTabUI("search");
+    ensureSearchHost();
+    await mountResultView({
+      eventRef,
+      triggerDate: params.get("trigger_date") || undefined,
+      party: params.get("party") || undefined,
+      courtId: params.get("court_id") || undefined,
+    });
+    return;
+  }
+
+  await activateTab(readTabFromUrl());
+}
+
 document.addEventListener("DOMContentLoaded", () => {
   initI18n();
   initSidebar();
-  showTab(readTabFromUrl());
   wireTabs();
+  void boot();
 });

frontend/src/procedures.tsx

@@ -148,8 +148,19 @@ export function renderProcedures(): string {
 
               <section className="procedures-panel" id="procedures-panel-search" role="tabpanel"
                        aria-labelledby="procedures-tab-search" hidden>
-                <div className="procedures-panel-placeholder" data-i18n="procedures.panel.search.placeholder">
-                  Direktsuche folgt in U1.
+                {/* Mode A host. procedures.ts (re-)seeds the inner
+                    overhaul-root / mode-host structure each time the
+                    Direkt-suchen tab activates, then defers to
+                    mountModeA() / mountResultView() — the legacy
+                    Fristenrechner overhaul code runs unchanged inside
+                    this wrapper. U3 will replace the in-panel commit
+                    behaviour with a pivot into the shared linear-output
+                    region; for U1 the result view simply re-renders
+                    into the same root. */}
+                <div className="procedures-mode-a-host">
+                  <div className="fristen-overhaul-root" id="fristen-overhaul-root">
+                    <div id="fristen-overhaul-mode-host"></div>
+                  </div>
                 </div>
               </section>