Add user avatar upload to account settings and nav bar

- Add `uploadAvatarAction` and `removeAvatarAction` server actions;
  store files under `DATA_DIR/avatars/{userId}.{ext}` via Bun.write
- Add `GET /api/avatars/[userId]` route that scans the avatar dir with
  Bun.Glob and serves with immutable Cache-Control (cache-busted via
  query param)
- Refactor AccountSection with hover overlay, AnimatePresence fade,
  and file input; click avatar to upload (no image) or remove (has image)
- Replace NavBar user initial badge with Avatar + DropdownMenu showing
  name, email, settings link, and sign-out
- Thread `userEmail` and `userImage` props through AuthenticatedShell
  → NavBar and SettingsPage → AccountSection

app/(pages)/settings/page.tsx

@@ -177,6 +177,7 @@ export default async function SettingsPage() {
         user={{
           name: session.user.name,
           email: session.user.email,
+          image: session.user.image || undefined,
           createdAt: session.user.createdAt.toISOString(),
           role: session.user.role ?? undefined,
         }}