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/auth/LoginPage.tsx

@@ -61,7 +61,7 @@ export function LoginPage() {
   const isDisabled = isLoading || retryAfter > 0;
 
   return (
-    <div className="min-h-screen flex items-center justify-center bg-surface-base relative">
+    <div className="min-h-full flex items-center justify-center bg-surface-base relative">
       <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top,rgba(124,108,246,0.06)_0%,transparent_50%)]" />
       <div className="w-full max-w-[480px] bg-surface-elevated rounded-md p-8 shadow-elevation-high relative z-10">
         <div className="text-center mb-6">

packages/web/src/components/auth/RegisterPage.tsx

@@ -236,7 +236,7 @@ export function RegisterPage() {
   const isDisabled = isRegistering || retryAfter > 0;
 
   return (
-    <div className="min-h-screen flex items-center justify-center bg-surface-base relative">
+    <div className="min-h-full flex items-center justify-center bg-surface-base relative">
       <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top,rgba(124,108,246,0.06)_0%,transparent_50%)]" />
       <div className="w-full max-w-[480px] bg-surface-elevated rounded-md p-8 shadow-elevation-high relative z-10 overflow-hidden">
         {/* Progress dots */}