fix: handle object-shaped validation errors in KB commit error toast

The commit endpoint returns {message, validation_errors} when
validation fails, but the error handler was passing the whole object
to toast.error(), crashing React with "Objects are not valid as a
React child".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/KBAcceleratorPage.tsx

@@ -140,8 +140,14 @@ export default function KBAcceleratorPage() {
       // Refresh quota
       kbAcceleratorApi.getQuota().then(setQuota).catch(() => {})
     } catch (err: unknown) {
-      const message = (err as { response?: { data?: { detail?: string } } })?.response?.data?.detail ?? 'Commit failed'
-      toast.error(message)
+      const detail = (err as { response?: { data?: { detail?: string | { message?: string; validation_errors?: string[] } } } })?.response?.data?.detail
+      if (typeof detail === 'object' && detail !== null) {
+        const msg = detail.message || 'Commit failed'
+        const errors = detail.validation_errors
+        toast.error(errors?.length ? `${msg}\n${errors.join('\n')}` : msg)
+      } else {
+        toast.error(typeof detail === 'string' ? detail : 'Commit failed')
+      }
     } finally {
       setLoading(false)
     }