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/UpdateToast.tsx

@@ -57,22 +57,22 @@ export function UpdateToast() {
     <div
       role="status"
       aria-live="polite"
-      className="fixed bottom-6 left-6 z-50 w-80 overflow-hidden rounded-2xl border border-white/10 bg-ink-900/95 p-4 shadow-2xl backdrop-blur-xl"
+      className="fixed bottom-6 left-6 z-50 w-80 overflow-hidden rounded-2xl border border-line bg-surface-3 p-4 shadow-2xl"
     >
       <div className="flex items-start gap-3">
-        <div className="flex h-9 w-9 shrink-0 items-center justify-center rounded-full bg-brand-500/25 text-brand-200 ring-1 ring-brand-400/30">
+        <div className="flex h-9 w-9 shrink-0 items-center justify-center rounded-full bg-accent/20 text-accent">
           <SparklesIcon className="h-4 w-4" />
         </div>
         <div className="min-w-0 flex-1">
-          <p className="text-sm font-semibold text-white">
+          <p className="text-sm font-semibold text-fg">
             {t('app:update.available', { defaultValue: 'Update verfügbar' })}
             {state.version ? ' · v' + state.version : ''}
           </p>
           {state.notes && (
-            <p className="mt-0.5 line-clamp-3 text-xs text-neutral-400">{state.notes}</p>
+            <p className="mt-0.5 line-clamp-3 text-xs text-fg-muted">{state.notes}</p>
           )}
           {state.error && (
-            <p className="mt-1 text-xs text-rose-300">{state.error}</p>
+            <p className="mt-1 text-xs text-rose-500 dark:text-rose-300">{state.error}</p>
           )}
         </div>
         {!installing && (
@@ -80,7 +80,7 @@ export function UpdateToast() {
             type="button"
             onClick={() => setDismissed(true)}
             aria-label={t('common:close', { defaultValue: 'Schließen' })}
-            className="inline-flex h-6 w-6 cursor-pointer items-center justify-center rounded-md text-neutral-400 transition hover:bg-white/10 hover:text-neutral-100"
+            className="inline-flex h-6 w-6 cursor-pointer items-center justify-center rounded-md text-fg-muted transition hover:bg-surface-2 hover:text-fg"
           >
             <XIcon className="h-3.5 w-3.5" />
           </button>
@@ -90,7 +90,7 @@ export function UpdateToast() {
         type="button"
         onClick={() => void handleInstall()}
         disabled={installing}
-        className="mt-3 inline-flex w-full cursor-pointer items-center justify-center gap-2 rounded-lg bg-gradient-to-br from-brand-400 to-brand-600 px-3 py-2 text-sm font-semibold text-white transition hover:from-brand-300 hover:to-brand-500 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-400/60 disabled:cursor-not-allowed disabled:opacity-60"
+        className="mt-3 inline-flex w-full cursor-pointer items-center justify-center gap-2 rounded-lg bg-accent px-3 py-2 text-sm font-semibold text-accent-fg transition hover:brightness-110 focus:outline-none focus-visible:ring-2 focus-visible:ring-accent/60 disabled:cursor-not-allowed disabled:opacity-60"
       >
         {installing ? (
           <>