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/components/Message/User.vue

@@ -8,7 +8,7 @@ defineProps<{
 
 <template>
     <div class="flex flex-col gap-2 max-w-full bg-[var(--bg-container)] py-2 px-3 rounded-xl">
-        <MarkdownRenderer :finished="true" :content="message.content!" :id="message.id" />
+        <MarkdownRenderer v-if="message.content" :finished="true" :content="message.content" :id="message.id" />
         <div v-if="message.attachments && message.attachments.length > 0" class="flex flex-col gap-2">
             <div v-for="attachment in message.attachments" :key="attachment.id"
                 class="flex flex-wrap items-center gap-2">