feat(chat): add message editing & regeneration (M8.1)

Pencil icon on hover for user messages to edit inline. Regenerate
icon on assistant messages to re-send from the same prompt. Editing
clears all subsequent messages and triggers a fresh AI response.

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

packages/core/src/types/message.ts

@@ -21,6 +21,8 @@ export interface Message {
   status?: 'sending' | 'sent' | 'delivered' | 'read' | 'error'
   /** Web search results (articles, links) attached when query used web search */
   webResults?: WebSearchResult[]
+  /** Timestamp of last edit (set when user edits a sent message) */
+  editedAt?: number
 }
 
 export interface Reaction {