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/components/SearchBar.css

@@ -0,0 +1,105 @@
+/* ---------------------------------------------------------------------------
+   SearchBar — find-in-scrollback overlay.
+
+   Positioned absolutely inside XtermPane's container div (which must be
+   position: relative).  Sits at the top-right of the pane, z-index 10 so it
+   floats above the xterm canvas but below any app-level modals (z-index 100).
+   Colour palette matches Palette.css / Help.css: #181818 surface, #2a2a2a
+   borders, #e6e6e6 text, #1a3a5c accent.
+--------------------------------------------------------------------------- */
+
+.search-bar {
+  position: absolute;
+  top: 4px;
+  right: 4px;
+  z-index: 10;
+  display: flex;
+  align-items: center;
+  gap: 3px;
+  background: #181818;
+  border: 1px solid #2a2a2a;
+  border-radius: 5px;
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.55);
+  padding: 3px 4px;
+  font-family: "Cascadia Mono", "JetBrains Mono", "Consolas", monospace;
+  font-size: 12px;
+  color: #e6e6e6;
+}
+
+.search-input {
+  font: inherit;
+  font-size: 12px;
+  color: #e6e6e6;
+  background: #1f1f1f;
+  border: 1px solid #2a2a2a;
+  border-radius: 3px;
+  padding: 3px 7px;
+  outline: none;
+  width: 180px;
+  caret-color: #e6e6e6;
+}
+
+.search-input:focus {
+  border-color: #1a3a5c;
+  box-shadow: 0 0 0 1px #1a3a5c;
+}
+
+.search-input::placeholder {
+  color: #555;
+}
+
+/* Toggle buttons (Aa / .*) */
+.search-toggle {
+  font: inherit;
+  font-size: 11px;
+  background: transparent;
+  border: 1px solid #2a2a2a;
+  border-radius: 3px;
+  color: #888;
+  padding: 2px 5px;
+  cursor: pointer;
+  line-height: 1;
+  transition: background 0.1s, color 0.1s;
+}
+
+.search-toggle:hover,
+.search-toggle[aria-pressed="true"] {
+  background: #1a3a5c;
+  border-color: #1a5c8a;
+  color: #cce6ff;
+}
+
+/* Prev / Next navigation arrows */
+.search-nav {
+  font: inherit;
+  font-size: 13px;
+  background: transparent;
+  border: 1px solid #2a2a2a;
+  border-radius: 3px;
+  color: #aaa;
+  padding: 1px 6px;
+  cursor: pointer;
+  line-height: 1;
+}
+
+.search-nav:hover {
+  background: #2a2a2a;
+  color: #e6e6e6;
+}
+
+/* Close button */
+.search-close {
+  background: transparent;
+  border: none;
+  color: #666;
+  font-size: 16px;
+  line-height: 1;
+  padding: 1px 5px;
+  cursor: pointer;
+  border-radius: 3px;
+}
+
+.search-close:hover {
+  background: #2a2a2a;
+  color: #ddd;
+}