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/layout/SpaceSidebar.tsx

@@ -13,6 +13,7 @@ import { TransferOwnershipModal } from '../modals/TransferOwnershipModal';
 import type { SpaceLayoutItem, SpaceFolder } from '@backspace/shared';
 
 import { getSpaceGradient, HOME_GRADIENT } from '../../utils/gradients';
+import { isElectronMac } from '../../platform/platform';
 import { useFloatingPosition } from '../../hooks/useFloatingPosition';
 
 // ─── Resolved layout types ─────────────────────────────────────────────────
@@ -1247,7 +1248,7 @@ export function SpaceSidebar() {
   }, [openFolderId, resolvedLayout]);
 
   return (
-    <nav data-pip-obstacle="left" className="w-[72px] bg-surface-base flex flex-col items-center py-3 overflow-y-auto flex-shrink-0 no-scrollbar select-none md:fixed md:inset-y-0 md:left-0 md:z-[100] md:glass-strip" style={{ paddingBottom: floatingPanelHeight + 24 }} onDragOver={(e) => { if (dragState) e.preventDefault(); }} onDrop={handleDrop}>
+    <nav data-pip-obstacle="left" className="w-[72px] bg-surface-base flex flex-col items-center py-3 overflow-y-auto flex-shrink-0 no-scrollbar select-none md:fixed md:inset-y-0 md:left-0 md:z-[100] md:glass-strip" style={{ paddingBottom: floatingPanelHeight + 24, ...(isElectronMac() ? { top: '32px' } : {}) }} onDragOver={(e) => { if (dragState) e.preventDefault(); }} onDrop={handleDrop}>
       <SidebarItem
         id="@me"
         name="Direct Messages"