fix(frontend): surface upload API error details (#1113)

Preserve backend upload/list/delete error details in the frontend API layer so users see the actual server failure instead of a generic message.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/uploads/api.ts

@@ -29,6 +29,16 @@ export interface ListFilesResponse {
   count: number;
 }
 
+async function readErrorDetail(
+  response: Response,
+  fallback: string,
+): Promise<string> {
+  const error = await response
+    .json()
+    .catch(() => ({ detail: fallback }));
+  return error.detail ?? fallback;
+}
+
 /**
  * Upload files to a thread
  */
@@ -51,10 +61,7 @@ export async function uploadFiles(
   );
 
   if (!response.ok) {
-    const error = await response
+    throw new Error(await readErrorDetail(response, "Upload failed"));
-      .json()
-      .catch(() => ({ detail: "Upload failed" }));
-    throw new Error(error.detail ?? "Upload failed");
   }
 
   return response.json();
@@ -71,7 +78,9 @@ export async function listUploadedFiles(
   );
 
   if (!response.ok) {
-    throw new Error("Failed to list uploaded files");
+    throw new Error(
+      await readErrorDetail(response, "Failed to list uploaded files"),
+    );
   }
 
   return response.json();
@@ -92,7 +101,7 @@ export async function deleteUploadedFile(
   );
 
   if (!response.ok) {
-    throw new Error("Failed to delete file");
+    throw new Error(await readErrorDetail(response, "Failed to delete file"));
   }
 
   return response.json();