fix: patch memory leaks in event hooks and strengthen type guard

Fix three bugs found during code review:

1. useTauriEvent/useTauriEvents: If the component unmounts before the
   async listen() promise resolves, the unlisten function was lost,
   leaking the event subscription. Added a cancelled flag to call
   unlisten immediately when the promise resolves after cleanup.

2. useTauriEvents: The handlers object was used directly as a useEffect
   dependency, causing re-subscription on every render when callers
   pass an inline object literal. Replaced with a useRef for handler
   stability and a derived eventNames string as the dependency.

3. isMcError type guard: Only checked property existence via 'in'
   operator, not property types. An object with wrong-typed properties
   (e.g. code: 42) would pass the guard. Now validates that code and
   message are strings and recoverable is boolean.

4. AppShell global shortcut listener: Same race condition as (1), plus
   missing .catch() on the listen promise could produce unhandled
   rejections.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/AppShell.tsx

@@ -38,14 +38,30 @@ export function AppShell(): React.ReactElement {
 
   // Listen for global shortcut events from the Rust backend
   useEffect(() => {
-    const unlisten = listen<string>("global-shortcut-triggered", (event) => {
+    let cancelled = false;
+    let unlisten: (() => void) | undefined;
+
+    listen<string>("global-shortcut-triggered", (event) => {
       if (event.payload === "quick-capture") {
         useCaptureStore.getState().open();
       }
-    });
+    })
+      .then((fn) => {
+        if (cancelled) {
+          fn();
+        } else {
+          unlisten = fn;
+        }
+      })
+      .catch((err: unknown) => {
+        console.warn("[AppShell] Failed to subscribe to global shortcut:", err);
+      });
 
     return () => {
-      unlisten.then((fn) => fn());
+      cancelled = true;
+      if (unlisten) {
+        unlisten();
+      }
     };
   }, []);