perf: migrate chat rendering to virtua (#1651)

* refactor: migrate chat history virtualization to virtua

* refactor: render loaded chat history directly

* refactor: finish virtua migration

* perf: defer tool body rendering

* perf: queue deferred tool body mounts

* perf: quiet and defer markdown file probes

* perf: defer markdown code highlighting

* perf: stabilize markdown plugin lists

* perf: defer mermaid markdown rendering

* perf: delay markdown file reference annotation

* perf: attach markdown table listeners on demand

* perf: trim markdown render overhead

packages/web/server/lib/fs/routes.js

@@ -699,6 +699,7 @@ export const registerFsRoutes = (app, dependencies) => {
 
   app.get('/api/fs/stat', async (req, res) => {
     const filePath = typeof req.query.path === 'string' ? req.query.path.trim() : '';
+    const optional = req.query.optional === 'true';
     if (!filePath) {
       return res.status(400).json({ error: 'Path is required' });
     }
@@ -740,6 +741,9 @@ export const registerFsRoutes = (app, dependencies) => {
     } catch (error) {
       const err = error;
       if (err && typeof err === 'object' && err.code === 'ENOENT') {
+        if (optional) {
+          return res.json({ path: filePath, exists: false });
+        }
         return res.status(404).json({ error: 'File not found' });
       }
       if (err && typeof err === 'object' && err.code === 'EACCES') {