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- Remove outdated macOS audio block (now supported via ScreenCaptureKit)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/package.json

@@ -20,7 +20,7 @@
     "electron-updater": "^6.3.0"
   },
   "devDependencies": {
-    "electron": "^33.2.0",
+    "electron": "^40.0.0",
     "electron-builder": "^25.1.8",
     "typescript": "^5.7.2"
   }