feat: redesign + avatars + theme + call presence fixes (v0.6.0)

Visual:
- Light/dark theme via ThemeContext + CSS vars
- New design tokens (surface/fg/accent/line/etc.) across all pages
- Reusable Avatar component (img + letter fallback) wired into UserBar,
  ConversationHeader, ChatsPage, FriendsPage, GroupInfoPanel, IncomingCallPanel

Calls:
- Split call UI: IncomingCallPanel, InCallPanel, CallControls,
  CallParticipantTile, ScreenShareViewer
- Active speaker hook (useActiveSpeakers)
- Fix: ActiveCallBanner stayed hidden after hangup while peers in room.
  - useCallPresence: bind presence callbacks only when we own subscribe
    (Supabase forbids .on() after .subscribe() on shared dedup'd channels)
  - useCallPresence: never removeChannel — channel is shared with CallContext
    so tearing it down on ConversationHeader unmount killed live tracking
  - ActiveCallBanner: lastCallConversationId fallback so banner shows
    instantly after hangup, auto-dismiss when room confirmed empty
- Drop unused useAnyActiveCall

Bump tauri version 0.5.0 -> 0.6.0

apps/desktop/src/components/AppShell.tsx

@@ -21,7 +21,7 @@ export function AppShell() {
   }, [session?.user.id, device?.id]);
 
   return (
-    <div className="relative flex min-h-screen overflow-hidden bg-ink-950 text-neutral-100">
+    <div className="relative flex min-h-screen overflow-hidden bg-surface text-fg">
       <ShellBackground />
       <div className="relative z-10 flex min-h-screen w-full">
         <Sidebar />
@@ -36,11 +36,11 @@ export function AppShell() {
   );
 }
 
-// Calmer than the auth-screen background — full-bleed grid + 2 large blobs.
-// No animation here so message lists stay readable.
+// Subtle ambient background — only renders in dark mode where the app's
+// visual language expects depth. Light mode stays clean and flat.
 function ShellBackground() {
   return (
-    <div aria-hidden="true" className="pointer-events-none absolute inset-0">
+    <div aria-hidden="true" className="pointer-events-none absolute inset-0 hidden dark:block">
       <div className="bg-grid absolute inset-0 opacity-[0.18]" />
       <div className="absolute -left-32 top-1/4 h-[420px] w-[420px] rounded-full bg-brand-500/20 blur-3xl" />
       <div className="absolute -right-32 bottom-0 h-[420px] w-[420px] rounded-full bg-fuchsia-500/10 blur-3xl" />