feat: Electron screen share picker, instance selector, and system audio loopback

- Custom screen share picker for Electron (ScreenSharePicker.tsx) with
  Aether Drift design: glass-bubble footer, adaptive grid, pill tabs,
  border-based selection (avoids overflow clipping), hover brightness
- Instance URL picker so Electron connects to any Backspace server
- System audio loopback on Windows/Linux via desktopCapturer callback
- macOS: video-only callback (OS blocks system audio capture)
- IPC bridge for screen source enumeration and selection
- Purge stale service worker caches on Electron launch

packages/web/src/components/voice/VoiceControls.tsx

@@ -59,6 +59,7 @@ export function VoiceControls() {
 
   const handleScreenShare = async () => {
     const room = getActiveRoom();
+    console.log('[SS] handleScreenShare clicked, room:', !!room, 'isScreenSharing:', isScreenSharing);
     if (!room) return;
     try {
       if (!isScreenSharing) {