feat(web): add favicon for CLI Web

Copy favicon.ico from chatlab.fun and serve it via Vite's publicDir
(src/public/). Update index.html to reference the standard /favicon.ico
path; the previous /assets/imgs/favicon.ico path had no backing file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/index.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8" />
     <title>ChatLab</title>
     <!-- https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP -->
-    <link rel="icon" href="/assets/imgs/favicon.ico" />
+    <link rel="icon" href="/favicon.ico" />
   </head>
 
   <body>