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>

package.json

@@ -20,6 +20,8 @@
     "@tauri-apps/plugin-opener": "^2.0.0",
     "@xterm/addon-canvas": "^0.7.0",
     "@xterm/addon-fit": "^0.10.0",
+    "@xterm/addon-search": "^0.15.0",
+    "@xterm/addon-unicode11": "^0.8.0",
     "@xterm/addon-web-links": "^0.12.0",
     "@xterm/xterm": "^5.5.0",
     "react": "^18.3.0",