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/desktop/src/preload.ts

@@ -50,6 +50,14 @@ contextBridge.exposeInMainWorld('backspace', {
     ipcRenderer.on('deep-link', (_event, url) => callback(url));
   },
 
+  // Screen share picker coordination
+  onScreenShareSources: (callback: (sources: unknown[]) => void) => {
+    ipcRenderer.on('screen-share-sources', (_event, sources) => callback(sources));
+  },
+  selectScreenSource: (sourceId: string | null, shareAudio?: boolean) => {
+    ipcRenderer.send('screen-share-selected', sourceId, shareAudio ?? true);
+  },
+
   // Instance URL management
   getInstanceUrl: () => ipcRenderer.invoke('get-instance-url'),
   setInstanceUrl: (url: string) => ipcRenderer.invoke('set-instance-url', url),