feat(calls): system-wide PTT/deafen on desktop via the native key poll (cinny-desktop #2)

PTT and deafen were DOM key handlers and only fired while Lotus (or the EC
iframe) had focus — alt-tab into a fullscreen game and the voice controls
stopped working.

- useCallHotkeys: while a call is joined and the new device-local
  `globalCallHotkeys` setting is on, register {ptt, deafen} bindings with
  the desktop (`set_global_hotkeys`, cleared on leave) and act on its
  `lotus-global-hotkey` press/release events. Events are ignored while
  `document.hasFocus()` so the DOM handlers keep owning the in-focus case
  (editable-field and interactive-element checks, no double toggles). PTT
  press engages the mic exactly like the DOM path (pttActive set before
  unmute), release restores; the existing blur/focus release covers a hold
  that spans a focus change. Same modifier rules as the DOM path
  (`shouldActOnGlobalHotkey`, tested).
- Settings → Calls: "Hotkeys Work Outside the Window" toggle, Tauri only.
- settingsSync: `globalCallHotkeys` is device-local (never synced).
- LOTUS_FEATURES: desktop section entry.

Native side lands in cinny-desktop (src-tauri/src/native/hotkeys.rs).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/settingsSync.ts

@@ -27,6 +27,7 @@ export type SyncedSettingsContent = {
 // another.
 export const DEVICE_LOCAL_KEYS: ReadonlySet<keyof Settings> = new Set<keyof Settings>([
   'settingsSync',
+  'globalCallHotkeys',
   'pageZoom',
   'mediaAutoLoad',
   'pauseAnimations',