refactor: replace focusedIndex with activeChildId

- Replace focusedIndex integer with activeChildId string in messages schema
- Add migration to convert existing data and drop focused_index column
- Add  hooks for providers.updatedAt and messageParts.lastUpdatedAt
- Add file size column to files table
- Update message navigation to use child ID-based lookup
- Simplify delete logic by removing focusedIndex math
- Fix file delete endpoint filename ([id[ -> [id])

app/pages/agent/[id]/index.vue

@@ -30,7 +30,6 @@ const handleSubmit = async (message: BaseMessage, model: ModelWithProvider | nul
     }
 
     console.log("handleSubmit", message);
-    if (!message.content) return;
 
     pendingMessage.value = {
         id: '',
@@ -43,7 +42,7 @@ const handleSubmit = async (message: BaseMessage, model: ModelWithProvider | nul
         parentMessageId: null,
         children: [],
         generationId: null,
-        focusedIndex: null,
+        activeChildId: null,
         attachments: [],
         deleted: false,
         createdAt: new Date(),