Fix error alerts showing "[object Object]" instead of the real message

FastAPI's `detail` field is a plain string for handler-raised errors, but
an array of {loc, msg, type} objects for its own automatic 422 request
validation errors. new Error(detail) silently stringified that array into
"[object Object]", hiding the actual validation failure from the user
(e.g. when adding an Odoo connection). Normalizes both shapes into one
readable message, and also guards against non-JSON error responses.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/lib/api.ts

@@ -1,12 +1,41 @@
 const BASE = import.meta.env.VITE_API_URL ?? 'https://odoo-mcp.thedomainnest.com';
 
+// FastAPI's `detail` field is a plain string for handler-raised HTTPExceptions,
+// but an array of {loc, msg, type} objects for its own automatic request
+// validation errors (422s) — normalize both into one readable string instead
+// of letting `new Error(x)` silently stringify an array/object into
+// "[object Object]".
+function errorDetailToMessage(detail: unknown): string {
+  if (typeof detail === 'string') return detail;
+  if (Array.isArray(detail)) {
+    return detail
+      .map((e) => {
+        if (e && typeof e === 'object' && 'msg' in e) {
+          const loc = Array.isArray((e as { loc?: unknown[] }).loc)
+            ? (e as { loc: unknown[] }).loc.filter((p) => p !== 'body').join('.')
+            : '';
+          return loc ? `${loc}: ${(e as { msg: string }).msg}` : (e as { msg: string }).msg;
+        }
+        return String(e);
+      })
+      .join('; ');
+  }
+  return 'Request failed';
+}
+
 async function req<T>(path: string, opts?: RequestInit): Promise<T> {
   const res = await fetch(`${BASE}${path}`, {
     headers: { 'Content-Type': 'application/json', ...opts?.headers },
     ...opts,
   });
-  const data = await res.json();
+  let data: unknown;
-  if (!res.ok) throw new Error(data.detail ?? 'Request failed');
+  try {
+    data = await res.json();
+  } catch {
+    if (!res.ok) throw new Error(`Request failed (HTTP ${res.status})`);
+    throw new Error('Server returned an unexpected (non-JSON) response.');
+  }
+  if (!res.ok) throw new Error(errorDetailToMessage((data as { detail?: unknown })?.detail));
   return data as T;
 }