feat: Add Picture-in-Picture floating video + fix local track visibility

- New PictureInPicture component: draggable floating window that shows
  active stream when navigating away from voice channel, with LIVE badge,
  snap-to-edge, click-to-navigate-back
- Fix VoiceUser to check actual track presence instead of boolean flags
  for video rendering
- Add LocalTrackPublished/LocalTrackUnpublished events to useLiveKit
  so local screen share and camera show immediately in VoiceGrid
- Add pipCollapsed state to uiStore

packages/web/src/components/layout/AppLayout.tsx

@@ -14,6 +14,7 @@ import { UserSettingsModal } from '../modals/UserSettings';
 import { ServerSettingsModal } from '../modals/ServerSettings';
 import { NewDmModal } from '../modals/NewDmModal';
 import { IncomingCallModal } from '../voice/IncomingCallModal';
+import { PictureInPicture } from '../voice/PictureInPicture';
 import { UserProfilePopout } from '../ui/UserProfilePopout';
 import { useAuth } from '../../hooks/useAuth';
 import { useWebSocket } from '../../hooks/useWebSocket';
@@ -161,6 +162,7 @@ export function AppLayout() {
       <NewDmModal />
       <IncomingCallModal />
       <ImagePreview />
+      <PictureInPicture />
 
       {/* User Profile Popout */}
       {userProfilePopout.user && userProfilePopout.position && (