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/components/ui/SwUpdatePrompt.tsx

@@ -1,13 +1,10 @@
 import { useRegisterSW } from 'virtual:pwa-register/react';
 import { useEffect } from 'react';
-import { isElectron } from '../../platform/platform';
 
 export function SwAutoUpdate() {
-  const inElectron = isElectron();
-
   useRegisterSW({
     onRegisteredSW(_swUrl, registration) {
-      if (!registration || inElectron) return;
+      if (!registration) return;
       setInterval(() => {
         registration.update();
       }, 60_000);
@@ -15,12 +12,11 @@ export function SwAutoUpdate() {
   });
 
   useEffect(() => {
-    if (inElectron) return;
     if (!navigator.serviceWorker) return;
     const onControllerChange = () => window.location.reload();
     navigator.serviceWorker.addEventListener('controllerchange', onControllerChange);
     return () => navigator.serviceWorker.removeEventListener('controllerchange', onControllerChange);
-  }, [inElectron]);
+  }, []);
 
   return null;
 }