fix: restore setDisplayMediaRequestHandler with useSystemPicker

Electron requires setDisplayMediaRequestHandler for getDisplayMedia() to
work — removing it broke screen sharing entirely. Restored the handler
with useSystemPicker: true, which on macOS 15+ uses the native system
picker (honoring restrictOwnAudio), while Windows/Linux fall back to
the custom picker with the shareAudio toggle for echo control.

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;
@@ -26,6 +34,10 @@ 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, shareAudio?: boolean) => void;
+
   // Instance URL management
   getInstanceUrl: () => Promise<string | null>;
   setInstanceUrl: (url: string) => Promise<void>;