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/app/src/stores/chat.ts

@@ -244,6 +244,28 @@ export const useChatStore = defineStore('chat', () => {
     }
   }
 
+  /** Update a message's content (for editing) */
+  function updateMessageContent(conversationId: string, messageId: string, newContent: string) {
+    const conv = conversations.value.get(conversationId)
+    if (!conv) return
+    const msg = conv.messages.find((m: { id: string }) => m.id === messageId)
+    if (msg) {
+      msg.content = newContent
+      msg.editedAt = Date.now()
+      conv.updatedAt = Date.now()
+      debouncedIDBSave(conv)
+    }
+  }
+
+  /** Delete all messages after the given index (inclusive of afterIndex) */
+  function deleteMessagesAfter(conversationId: string, afterIndex: number) {
+    const conv = conversations.value.get(conversationId)
+    if (!conv) return
+    conv.messages.splice(afterIndex)
+    conv.updatedAt = Date.now()
+    debouncedIDBSave(conv)
+  }
+
   return {
     conversations,
     activeConversationId,
@@ -263,5 +285,7 @@ export const useChatStore = defineStore('chat', () => {
     toggleChatCollapse,
     setActiveConversation,
     deleteConversation,
+    updateMessageContent,
+    deleteMessagesAfter,
   }
 })