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/styles/globals.css

@@ -455,3 +455,7 @@
   .animate-call-button-glow { animation: none !important; }
   .call-refraction::after { animation: none !important; opacity: 0; }
 }
+
+/* ── Electron window drag regions ── */
+.titlebar-drag { -webkit-app-region: drag; }
+.titlebar-no-drag { -webkit-app-region: no-drag; }