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/platform/platform.ts

@@ -2,6 +2,10 @@ export function isElectron(): boolean {
   return typeof window !== 'undefined' && typeof window.backspace !== 'undefined';
 }
 
+export function isElectronMac(): boolean {
+  return isElectron() && window.backspace?.platform === 'darwin';
+}
+
 export function getElectronAPI(): BackspaceElectronAPI | null {
   return window.backspace ?? null;
 }