fix(ui): resolve avatar URL from MinIO; fall back to OAuth provider URL

Add resolveAvatarUrl(userId, storedValue) helper that tries MinIO first,
then falls back to the stored HTTP URL for OAuth users (Google/GitHub)
who have never uploaded a custom avatar.

Add getUserById() to pocketbase helpers for batch avatar resolution in
comments. Update all 6 call sites to use the new helper.

ui/src/lib/server/pocketbase.ts

@@ -541,6 +541,19 @@ export async function getUserByUsername(username: string): Promise<User | null>
 	return listOne<User>('app_users', `username="${username.replace(/"/g, '\\"')}"`);
 }
 
+/**
+ * Look up a user by their PocketBase record ID. Returns null if not found.
+ */
+export async function getUserById(id: string): Promise<User | null> {
+	const token = await getToken();
+	const res = await fetch(`${PB_URL}/api/collections/app_users/records/${encodeURIComponent(id)}`, {
+		headers: { Authorization: `Bearer ${token}` }
+	});
+	if (res.status === 404) return null;
+	if (!res.ok) return null;
+	return res.json() as Promise<User>;
+}
+
 /**
  * Look up a user by email. Returns null if not found.
  */