feat: avatar upload via presigned PUT URL flow

- Go scraper: add PresignAvatarUploadURL/PresignAvatarURL/DeleteAvatar to
  Store interface, implement on HybridStore+MinioClient, register
  GET /api/presign/avatar-upload/{userId} and /api/presign/avatar/{userId}
- SvelteKit: replace direct AWS S3 SDK in minio.ts with presign calls to
  the Go scraper; rewrite avatar +server.ts (POST=presign, PATCH=record key)
- iOS: rewrite uploadAvatar() as 3-step presigned PUT flow; refactor
  chip components into shared ChipButton in CommonViews.swift

ui/src/lib/server/pocketbase.ts

@@ -801,9 +801,13 @@ export async function revokeAllUserSessions(userId: string): Promise<void> {
  */
 export async function updateUserAvatarUrl(userId: string, avatarUrl: string): Promise<void> {
 	const token = await getToken();
-	await fetch(`${PB_URL}/api/collections/app_users/records/${userId}`, {
+	const res = await fetch(`${PB_URL}/api/collections/app_users/records/${userId}`, {
 		method: 'PATCH',
 		headers: { Authorization: `Bearer ${token}`, 'Content-Type': 'application/json' },
 		body: JSON.stringify({ avatar_url: avatarUrl })
 	});
+	if (!res.ok) {
+		const body = await res.text().catch(() => '');
+		throw new Error(`updateUserAvatarUrl failed: ${res.status} ${body}`);
+	}
 }