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/electron.d.ts

@@ -1,5 +1,13 @@
 /** Type augmentation for the Electron IPC bridge exposed by preload.ts */
 
+interface ElectronScreenSource {
+  id: string;                      // "screen:0:0" or "window:12345:0"
+  name: string;                    // "Entire Screen" or "Firefox"
+  thumbnailDataUrl: string;        // PNG data URL at 320×180
+  appIconDataUrl: string | null;   // App icon (windows only)
+  isScreen: boolean;               // true = display, false = window
+}
+
 interface BackspaceElectronAPI {
   // Platform info
   platform: NodeJS.Platform;
@@ -25,6 +33,15 @@ interface BackspaceElectronAPI {
 
   // Deep linking (Task 2.3)
   onDeepLink: (callback: (url: string) => void) => void;
+
+  // Screen share picker coordination
+  onScreenShareSources: (callback: (sources: ElectronScreenSource[]) => void) => void;
+  selectScreenSource: (sourceId: string | null) => void;
+
+  // Instance URL management
+  getInstanceUrl: () => Promise<string | null>;
+  setInstanceUrl: (url: string) => Promise<void>;
+  clearInstanceUrl: () => Promise<void>;
 }
 
 interface Window {