feat(call): mute + deafen global hotkeys with chord support (group E)

- New voiceHotkeys.ts storage module. Stores per-action bindings with
  modifier flags (Ctrl/Shift/Alt) so Discord-style chords like
  Ctrl+Shift+M work. Defaults match Discord — Ctrl+Shift+M mute,
  Ctrl+Shift+D deafen — but ship disabled to avoid surprise collisions.
- globalShortcut.ts grows registerGlobalShortcutPress /
  unregisterGlobalShortcut helpers that accept pre-formatted Tauri
  accelerator strings, since voiceHotkey chords can't be expressed by
  the existing codeToShortcut path (PTT-only single-key).
- CallContext registers the chords OS-wide while a call is active
  (connected | reconnecting) so the hotkeys work from any focused
  window. A window-keydown fallback handles the non-Tauri / denied
  registration case. Both unregister on call end.
- SettingsPage adds VoiceHotkeyControls (mute + deafen variants)
  with a chord-capture button that waits past modifier-only presses
  and binds to the first real key. Labels render as "Ctrl+Shift+M"
  consistently with the in-call PTT hint.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src/lib/globalShortcut.ts

@@ -48,6 +48,38 @@ export async function unregisterPttShortcut(code: string): Promise<void> {
   }
 }
 
+// Press-only global shortcut (for toggles like Mute/Deafen). Accepts an
+// already-formatted accelerator string (e.g. "CommandOrControl+Shift+M")
+// since these bindings may include modifier chords — the KeyboardEvent.code
+// variant used by PTT can't express that.
+export async function registerGlobalShortcutPress(
+  shortcut: string,
+  onPress: () => void,
+): Promise<boolean> {
+  try {
+    if (await isRegistered(shortcut)) {
+      await unregister(shortcut);
+    }
+    await register(shortcut, (event: ShortcutEvent) => {
+      if (event.state === 'Pressed') onPress();
+    });
+    return true;
+  } catch (err: unknown) {
+    console.warn('registerGlobalShortcutPress failed', { shortcut, err });
+    return false;
+  }
+}
+
+export async function unregisterGlobalShortcut(shortcut: string): Promise<void> {
+  try {
+    if (await isRegistered(shortcut)) {
+      await unregister(shortcut);
+    }
+  } catch (err: unknown) {
+    console.warn('unregisterGlobalShortcut failed', { shortcut, err });
+  }
+}
+
 // Detects whether we're running under Tauri. When running in a pure web
 // preview (vite dev in a browser without Tauri), importing the plugin still
 // works but calls fall through to window.__TAURI_INTERNALS__ which doesn't