bugfix: don't JSON.parse empty 200/201 bodies (0.19.1)

$(addMangaToCollection crashed when the backend returned 201/200 with no body — the shared client only short-circuited 204. Now any empty body returns undefined.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/api/client.ts

@@ -56,10 +56,18 @@ export async function request<T>(path: string, init?: RequestInit): Promise<T> {
         }
         throw new ApiError(res.status, code, message);
     }
+    // Any empty body (not just 204) returns undefined — the manga-add
+    // endpoint, for instance, signals create-vs-already-present via
+    // 201/200 with no body, and callers typed `request<void>` would
+    // otherwise blow up on `res.json()` parsing an empty string.
     if (res.status === 204) {
         return undefined as T;
     }
-    return (await res.json()) as T;
+    const text = await res.text();
+    if (!text) {
+        return undefined as T;
+    }
+    return JSON.parse(text) as T;
 }
 
 export type Manga = {