feat: responsive mobile layout with slide-over sidebar

On screens ≤600px the sidebar collapses off-screen. A hamburger button
in the message header opens it as a slide-over drawer with a dim
overlay; tapping a room/network or the ✕ button closes it. Desktop
layout is unchanged.

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

.gitignore

@@ -21,6 +21,7 @@ build-daemon.sh
 deploy-web.sh
 deploy-daemon.sh
 serve-web.sh
+push.sh
 web/public/config.js
 cmd/app/frontend/dist/*
 !cmd/app/frontend/dist/.gitkeep