fix(desktop): release keep-awake on unmount

Fixes #76

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/hooks/useTauriCallPower.ts

@@ -14,5 +14,11 @@ export function useTauriCallPower(): void {
   const callEmbed = useAtomValue(callEmbedAtom);
   useEffect(() => {
     invokeTauri('set_call_active', { active: callEmbed !== undefined });
+    // #76 — if the shell unmounts while a call is still active (forced
+    // reload/HMR, or a future teardown path that doesn't end the call first),
+    // release the native keep-awake state instead of leaving it held forever.
+    return () => {
+      invokeTauri('set_call_active', { active: false });
+    };
   }, [callEmbed]);
 }