Add find-in-scrollback, unicode11, and keyboard pane navigation

Three xterm.js features, implemented together because they share the
XtermPane mount + the single attachCustomKeyEventHandler:

- Unicode 11: load @xterm/addon-unicode11, set activeVersion='11' after
  the canvas renderer so emoji/CJK/box-drawing widths stop drifting.
- Find in scrollback: @xterm/addon-search + a new per-pane SearchBar
  overlay (Ctrl+Shift+F to open, Enter/Shift+Enter next/prev, regex +
  case toggles, Esc to close & refocus). Overlay is an absolutely-
  positioned sibling in a position:relative wrapper so fit() is unaffected.
- Pane navigation: Ctrl+Alt+Arrow / Ctrl+Alt+HJKL (spatial neighbour via
  findNeighborInDirection) and Alt+1..9 (Nth leaf in walkLeaves order).
  XtermPane emits a NavigateIntent; App resolves the target leaf and sets
  it active, reusing the existing isActive->focusTrigger refocus chain.

All chords live in one attachCustomKeyEventHandler (xterm replaces the
handler on each call). Shortcuts added to shortcuts.ts (SoT for README +
Help), including the Alt+digit shell-conflict caveat. tsc clean apart
from the three not-yet-installed addon modules.

Needs pnpm install on the Windows host + runtime verification.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -98,7 +98,7 @@ import {
   presetTwoRows,
   presetTwoByTwo,
 } from "./lib/layout/tree";
-import { OrchestrationProvider, type Orchestration } from "./lib/layout/orchestration";
+import { OrchestrationProvider, type Orchestration, type NavigateIntent } from "./lib/layout/orchestration";
 import LeafPane from "./lib/layout/LeafPane";
 import Gutter from "./lib/layout/Gutter";
 import Notifications, { type Toast } from "./components/Notifications";
@@ -717,6 +717,36 @@ export default function App() {
     setActiveLeafId(leafId);
   }, []);
 
+  // navigateTo — called from XtermPane's attachCustomKeyEventHandler via
+  // LeafPane's onNavigate prop. Resolves the target leaf from the current
+  // layout tree and sets it active; the LeafPane isActive→focusTrigger
+  // effect then refocuses the xterm textarea automatically.
+  const navigateTo = useCallback((intent: NavigateIntent) => {
+    const currentTree = treeRef.current;
+    const currentActiveId = activeLeafId;
+
+    if (intent.kind === "direction") {
+      const layout = flattenLayout(currentTree);
+      if (!currentActiveId) {
+        const first = layout.leaves[0]?.leaf.id;
+        if (first) setActiveLeafId(first);
+        return;
+      }
+      const nextId = findNeighborInDirection(
+        layout.leaves,
+        currentActiveId,
+        intent.dir,
+      );
+      if (nextId) setActiveLeafId(nextId);
+    } else {
+      // intent.kind === "index"
+      const leaves = Array.from(walkLeaves(currentTree));
+      // Clamp: Alt+9 with 3 panes picks the 3rd pane.
+      const target = leaves[Math.min(intent.n, leaves.length) - 1];
+      if (target) setActiveLeafId(target.id);
+    }
+  }, [activeLeafId]); // treeRef is a ref — stable, intentionally not listed
+
   const openHostManager = useCallback(() => setHostManagerOpen(true), []);
   const closeHostManager = useCallback(() => setHostManagerOpen(false), []);
 
@@ -1242,6 +1272,7 @@ export default function App() {
       toggleMcpAllow,
       openHostManager,
       setActive,
+      navigateTo,
       registerPaneId,
       broadcastFrom,
       notify,
@@ -1266,6 +1297,7 @@ export default function App() {
       toggleMcpAllow,
       openHostManager,
       setActive,
+      navigateTo,
       registerPaneId,
       broadcastFrom,
       notify,