fix(KB): silent maybe-later error + redundant prompt-state refetches (PR #899 review)

- KbPolicyPromptBanner: add onError toast to maybeLaterMutation so a
    failed policy save surfaces to the user instead of looking like a
    broken button (banner would otherwise reappear on next chat open
    with no explanation).
  - KbPolicyPromptBanner: set staleTime: Infinity on the prompt-state
    query. For users who already picked a policy (the vast majority),
    the result is effectively immutable per session — the mutations
    invalidate the key when it actually changes.

admin/inertia/components/chat/KbPolicyPromptBanner.tsx

@@ -37,6 +37,7 @@ export default function KbPolicyPromptBanner() {
   const { data: promptState } = useQuery({
     queryKey: ['kbPolicyPromptState'],
     queryFn: () => api.getKbPolicyPromptState(),
+    staleTime: Infinity,
   })
 
   const indexNowMutation = useMutation({
@@ -72,6 +73,12 @@ export default function KbPolicyPromptBanner() {
       queryClient.invalidateQueries({ queryKey: ['kbPolicyPromptState'] })
       queryClient.invalidateQueries({ queryKey: ['ingestPolicy'] })
     },
+    onError: (error: any) => {
+      addNotification({
+        type: 'error',
+        message: error?.message || 'Could not save your choice. Try again.',
+      })
+    },
   })
 
   if (!promptState?.shouldPrompt) return null