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- Remove outdated macOS audio block (now supported via ScreenCaptureKit)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/desktop/electron-builder.yml

@@ -17,6 +17,9 @@ protocols:
       - backspace
 mac:
   category: public.app-category.social-networking
+  minimumSystemVersion: "12.0"
+  extendInfo:
+    NSAudioCaptureUsageDescription: "Backspace needs access to system audio to share sound during screen sharing."
   target:
     - dmg
     - zip