fix: tag color bug, FormData interceptor, falsy id check

- Fix tag using category_color instead of its own color when editing
- Add FormData auto-detection interceptor in client.ts (root cause of 422)
- Remove redundant Content-Type overrides from all 5 upload endpoints
- Fix falsy id check (id: 0 treated as null) with strict comparison
- Await invalidateQueries to prevent stale dropdown data

src/api/adminBroadcasts.ts

@@ -242,11 +242,7 @@ export const adminBroadcastsApi = {
     formData.append('file', file);
     formData.append('media_type', mediaType);
 
-    const response = await apiClient.post<MediaUploadResponse>('/cabinet/media/upload', formData, {
-      headers: {
-        'Content-Type': 'multipart/form-data',
-      },
-    });
+    const response = await apiClient.post<MediaUploadResponse>('/cabinet/media/upload', formData);
     return response.data;
   },
 };