feat(call): PTT, deafen label, camera default off, screenshare confirm, noise suppression setting

- Push to Talk: keydown/keyup binds mic to configurable key (default Space)
  with visual PTT indicator and key-binding UI in Settings > General > Calls
- Camera always defaults OFF on join; cameraOnJoin setting for explicit opt-in
- Deafen button tooltip corrected to Deafen/Undeafen instead of Turn Off/On Sound
- Screenshare confirmation dialog before broadcasting to call participants
- Noise suppression toggle wired from settings through CallEmbed URL params
- CallControl.setMicrophone() public method for programmatic mic control
- Calls settings section added to General settings page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/state/callPreferences.ts

@@ -28,10 +28,13 @@ export const makeCallPreferencesAtom = (userId: string): CallPreferencesAtom =>
     storeKey,
     (key) => {
       const v = getLocalStorageItem<CallPreferences>(key, DEFAULT_PREFERENCES);
-      return v;
+      // Never restore camera state — always start with camera off for privacy.
+      // Users can toggle it in the prescreen before joining.
+      return { ...v, video: false };
     },
     (key, value) => {
-      setLocalStorageItem(key, value);
+      // Don't persist video state — always resets to off on next load.
+      setLocalStorageItem(key, { ...value, video: false });
     }
   );