feat: in-app update checker + Spinner import for General settings

- Add useTauriUpdater hook (check_for_update / install_update commands)
- Add AppUpdates section to General settings (Tauri-only, hidden on web)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/hooks/useTauriUpdater.ts

@@ -0,0 +1,47 @@
+import { useState, useCallback } from 'react';
+
+type TauriInternals = { invoke: (cmd: string, args?: Record<string, unknown>) => Promise<unknown> };
+const tauriInvoke = (): TauriInternals['invoke'] | undefined =>
+  (window as unknown as { __TAURI_INTERNALS__?: TauriInternals }).__TAURI_INTERNALS__?.invoke;
+
+type UpdateStatus =
+  | { state: 'idle' }
+  | { state: 'checking' }
+  | { state: 'up-to-date' }
+  | { state: 'available'; version: string }
+  | { state: 'installing' }
+  | { state: 'error'; message: string };
+
+export function useTauriUpdater() {
+  const isTauri = !!tauriInvoke();
+  const [status, setStatus] = useState<UpdateStatus>({ state: 'idle' });
+
+  const check = useCallback(async () => {
+    const invoke = tauriInvoke();
+    if (!invoke) return;
+    setStatus({ state: 'checking' });
+    try {
+      const result = (await invoke('check_for_update')) as { available: boolean; version?: string };
+      setStatus(
+        result.available && result.version
+          ? { state: 'available', version: result.version }
+          : { state: 'up-to-date' },
+      );
+    } catch (e) {
+      setStatus({ state: 'error', message: String(e) });
+    }
+  }, []);
+
+  const install = useCallback(async () => {
+    const invoke = tauriInvoke();
+    if (!invoke) return;
+    setStatus({ state: 'installing' });
+    try {
+      await invoke('install_update');
+    } catch (e) {
+      setStatus({ state: 'error', message: String(e) });
+    }
+  }, []);
+
+  return { isTauri, status, check, install };
+}