feat: file upload retry, secure file tokens, UI polish

- Add HMAC-based file token auth for secure AI model file access
- Add file upload retry with exponential backoff (max 3 retries)
- File endpoint now requires session auth or signed token
- Support assistant role messages in chat input
- Optimistic UI for attachments on message send
- Verify topic ownership before allowing messages
- Switch web scraping to Firecrawl API
- Agent profile page layout fixes (proper flex overflow)
- Add quick switcher (Ctrl+K) to sidenav
- Clean up longcat.ts and stale comments

app/pages/agent/[id]/topic/[topicId].vue

@@ -54,6 +54,27 @@ const submitMessage = async (message: BaseMessage, model: ModelWithProvider | nu
     startGeneration(model);
 };
 
+const handleAddMessage = async (message: BaseMessage, role: 'user' | 'assistant', _model: ModelWithProvider | null) => {
+    inputValue.value = { content: '', fileIds: [] };
+
+    const res = await sendMessage(message, async () => {
+        await nextTick();
+        setTimeout(() => {
+            scrollToBottom('instant');
+        });
+    }, role);
+    if (!res.ok) {
+        console.error('Failed to add message:', res.error);
+        const chatInput = document.getElementById('chat') as HTMLInputElement | null;
+        if (chatInput) {
+            inputValue.value = message;
+            nextTick(() => {
+                chatInput.focus();
+            });
+        }
+    }
+};
+
 const handleRegenerate = async (message: Message) => {
     if (!agent.value!.defaultModelId) {
         console.error('No model selected');
@@ -292,8 +313,8 @@ console.log("full page render took", Date.now() - rootStart);
 
                 <div class="sticky bottom-0 z-10 bg-[var(--bg-surface)] pb-4 w-full rounded-t-2xl">
                     <ChatInput v-model="inputValue" class="[view-transition-name:chat-prompt] duration-150 ease-in-out"
-                        :loading="activeGeneration !== null" :agent="agent"
-                        :providers="providers?.filter(p => p.enabled)" @submit="submitMessage" @cancel="handleCancel"
+                        :loading="activeGeneration !== null" :allow-manual-role="true" :agent="agent"
+                        :providers="providers?.filter(p => p.enabled)" @submit="submitMessage" @add-message="handleAddMessage" @cancel="handleCancel"
                         @resize="handleResize" />
                 </div>
             </div>