fix: eliminate screen share audio feedback loop + upgrade Electron 33→40

Screen sharing with audio captured the app's own voice playback, causing
users to hear themselves echoed back. Fixed via two layers:

- Add restrictOwnAudio constraint (Chrome 141+/Chromium 144) to exclude
  the app's own audio from system audio capture
- Add shareAudio toggle so users can disable system audio entirely
- Remove outdated macOS audio block (now supported via ScreenCaptureKit)
- Upgrade Electron 33→40 (Chromium 130→144) so restrictOwnAudio works
  natively in the desktop app
- Add NSAudioCaptureUsageDescription for macOS 14.2+ audio capture
- Add GTK 3 fallback for Linux GNOME compatibility (Electron 36+)

packages/web/src/platform/electron.d.ts

@@ -36,7 +36,7 @@ interface BackspaceElectronAPI {
 
   // Screen share picker coordination
   onScreenShareSources: (callback: (sources: ElectronScreenSource[]) => void) => void;
-  selectScreenSource: (sourceId: string | null) => void;
+  selectScreenSource: (sourceId: string | null, shareAudio?: boolean) => void;
 
   // Instance URL management
   getInstanceUrl: () => Promise<string | null>;