Fix Uint8Array type error breaking Docker build

TypeScript 5.7+ makes Uint8Array generic over its buffer type.
Pass .buffer as ArrayBuffer to pushManager.subscribe() to fix
the type incompatibility with BufferSource.

Co-Authored-By: Claude Opus 4.6 (1M context) <[e-mail]>

web/src/hooks/usePushNotifications.ts

···
     const registration = await navigator.serviceWorker.ready;
     const subscription = await registration.pushManager.subscribe({
       userVisibleOnly: true,
-      applicationServerKey,
+      applicationServerKey: applicationServerKey.buffer as ArrayBuffer,
     });
 
     const json = subscription.toJSON();