feat(chat): add response feedback thumbs up/down (M9.5)

Thumbs up/down buttons on assistant messages (hover action bar). Feedback
stored per message, persisted in IDB, included in conversation exports.
Toggle to clear feedback. Color-coded: green for up, red for down.

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

packages/app/src/stores/chat.ts

@@ -222,6 +222,16 @@ export const useChatStore = defineStore('chat', () => {
     }
   }
 
+  function setMessageFeedback(conversationId: string, messageId: string, feedback: 'up' | 'down' | undefined) {
+    const conv = conversations.value.get(conversationId)
+    if (!conv) return
+    const msg = conv.messages.find((m: { id: string }) => m.id === messageId)
+    if (msg) {
+      msg.feedback = feedback
+      debouncedIDBSave(conv)
+    }
+  }
+
   function switchSide() {
     panelSide.value = panelSide.value === 'right' ? 'left' : 'right'
   }
@@ -348,6 +358,7 @@ export const useChatStore = defineStore('chat', () => {
     addMessage,
     appendToLastMessage,
     setMessageWebResults,
+    setMessageFeedback,
     switchSide,
     toggleChatCollapse,
     setActiveConversation,