FEAT-3: fix unhandled ApiError rejections (catch in form submits + dev unhandledrejection guard, ApiError carries path)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gerbil-manager-web/src/api/client.ts

@@ -10,11 +10,14 @@ export const API_BASE_URL: string =
 
 export class ApiError extends Error {
   readonly status: number | null
+  /** The request path that failed (for diagnostics/logging). */
+  readonly path?: string
 
-  constructor(message: string, status: number | null) {
+  constructor(message: string, status: number | null, path?: string) {
     super(message)
     this.name = 'ApiError'
     this.status = status
+    this.path = path
   }
 }
 
@@ -32,11 +35,11 @@ async function request<T>(path: string, init?: RequestInit): Promise<T> {
       ...init,
     })
   } catch {
-    throw new ApiError(de.api.errors.network, null)
+    throw new ApiError(de.api.errors.network, null, path)
   }
 
   if (!response.ok) {
-    throw new ApiError(errorMessageFor(response.status), response.status)
+    throw new ApiError(errorMessageFor(response.status), response.status, path)
   }
 
   if (response.status === 204) {