Add favicon

Link /assets/favicon.ico from every page and serve /favicon.ico without
requiring the access key (browsers request it on their own).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/index.js

@@ -67,6 +67,7 @@ const tables = new TableManager();
 
 // ---------- 認証不要 ----------
 app.use('/assets', express.static(path.join(PUBLIC, 'assets'), { index: false, dotfiles: 'deny' }));
+app.get('/favicon.ico', (req, res) => res.sendFile(path.join(PUBLIC, 'assets', 'favicon.ico')));
 app.get('/robots.txt', (req, res) => res.type('text/plain').send('User-agent: *\nDisallow: /\n'));
 app.get('/key', (req, res) => {
   if (auth.isAuthed(req)) return res.redirect(auth.safeNext(req.query.next));