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/core/src/types/message.ts

@@ -32,6 +32,8 @@ export interface Message {
   editedAt?: number
   /** Attached images (base64, max 4) for vision-capable models */
   images?: ImageAttachment[]
+  /** User feedback: 'up' = thumbs up, 'down' = thumbs down */
+  feedback?: 'up' | 'down'
 }
 
 export interface Reaction {