fix: revert UI and notify user when feedback submission fails (#237)

When feedback submission to the API fails, revert the optimistic
UI update and show a toast notification to inform the user.

Changes:
- Add toast import from sonner
- Change console.warn to console.error for proper logging
- Add toast.error() notification when API call fails
- Revert optimistic UI update by removing feedback from state

Previously, feedback submission failures were completely silent.
Users would see the thumbs-up/down visual feedback but their
feedback was never recorded. This creates a false sense that
the feedback was successfully submitted.

Now users are immediately notified when submission fails and
can retry their feedback.

components/chat-message-display.tsx

@@ -270,7 +270,14 @@ export function ChatMessageDisplay({
                 }),
             })
         } catch (error) {
-            console.warn("Failed to log feedback:", error)
+            console.error("Failed to log feedback:", error)
+            toast.error("Failed to record your feedback. Please try again.")
+            // Revert optimistic UI update
+            setFeedback((prev) => {
+                const next = { ...prev }
+                delete next[messageId]
+                return next
+            })
         }
     }