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/icons.tsx

@@ -336,6 +336,89 @@ export function MonitorStopIcon(props: IconProps) {
   );
 }
 
+export function SunIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <circle cx="12" cy="12" r="4" />
+      <path d="M12 2v2M12 20v2M4.93 4.93l1.41 1.41M17.66 17.66l1.41 1.41M2 12h2M20 12h2M4.93 19.07l1.41-1.41M17.66 6.34l1.41-1.41" />
+    </Base>
+  );
+}
+
+export function MoonIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <path d="M21 12.79A9 9 0 1 1 11.21 3a7 7 0 0 0 9.79 9.79Z" />
+    </Base>
+  );
+}
+
+export function GridIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <rect x="3" y="3" width="7" height="7" rx="1.5" />
+      <rect x="14" y="3" width="7" height="7" rx="1.5" />
+      <rect x="3" y="14" width="7" height="7" rx="1.5" />
+      <rect x="14" y="14" width="7" height="7" rx="1.5" />
+    </Base>
+  );
+}
+
+export function FocusIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <rect x="3" y="3" width="18" height="18" rx="2" />
+      <rect x="8" y="8" width="8" height="8" rx="1" />
+    </Base>
+  );
+}
+
+export function MaximizeIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <path d="M4 9V5a1 1 0 0 1 1-1h4" />
+      <path d="M20 9V5a1 1 0 0 0-1-1h-4" />
+      <path d="M4 15v4a1 1 0 0 0 1 1h4" />
+      <path d="M20 15v4a1 1 0 0 1-1 1h-4" />
+    </Base>
+  );
+}
+
+export function VideoIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <rect x="2" y="6" width="15" height="12" rx="2" />
+      <path d="m22 8-5 4 5 4V8Z" />
+    </Base>
+  );
+}
+
+export function CrownIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <path d="m3 7 4 4 5-6 5 6 4-4v10a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V7Z" />
+    </Base>
+  );
+}
+
+export function SendIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <path d="m3 11 18-8-8 18-2-8-8-2Z" />
+    </Base>
+  );
+}
+
+export function AddUserIcon(props: IconProps) {
+  return (
+    <Base {...props}>
+      <path d="M16 21v-2a4 4 0 0 0-4-4H6a4 4 0 0 0-4 4v2" />
+      <circle cx="9" cy="7" r="4" />
+      <path d="M19 8v6M22 11h-6" />
+    </Base>
+  );
+}
+
 export function LogoMark(props: IconProps) {
   return (
     <svg