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/Modal.tsx

@@ -35,22 +35,22 @@ export function Modal({ open, title, onClose, children, size = 'md' }: Props) {
       aria-modal="true"
       aria-label={title}
       onClick={onClose}
-      className="fixed inset-0 z-50 flex items-center justify-center bg-ink-950/80 p-6 backdrop-blur-sm animate-fade-in"
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/60 p-6 backdrop-blur-sm animate-fade-in"
     >
       <div
         onClick={(e) => e.stopPropagation()}
         className={
-          'relative w-full animate-slide-up rounded-2xl border border-white/10 bg-ink-900/95 shadow-xl backdrop-blur-xl ' +
+          'relative w-full animate-slide-up rounded-2xl border border-line bg-surface-3 shadow-xl ' +
           width
         }
       >
-        <header className="flex items-center justify-between border-b border-white/5 px-6 py-4">
-          <h2 className="font-display text-lg font-semibold text-white">{title}</h2>
+        <header className="flex items-center justify-between border-b border-line px-6 py-4">
+          <h2 className="font-display text-lg font-semibold text-fg">{title}</h2>
           <button
             type="button"
             onClick={onClose}
             aria-label="Close"
-            className="flex h-8 w-8 cursor-pointer items-center justify-center rounded-lg text-neutral-400 transition hover:bg-white/10 hover:text-neutral-100 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-400/40"
+            className="flex h-8 w-8 cursor-pointer items-center justify-center rounded-lg text-fg-muted transition hover:bg-surface-2 hover:text-fg focus:outline-none focus-visible:ring-2 focus-visible:ring-accent/40"
           >
             <XIcon className="h-4 w-4" />
           </button>