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>

web/src/pages/Chat.tsx

@@ -1,3 +1,4 @@
+import { useState } from 'react'
 import { Sidebar } from '../components/Sidebar'
 import { MessagePane } from '../components/MessagePane'
 import { FileBrowser } from '../components/FileBrowser'
@@ -5,10 +6,11 @@ import { useWaste } from '../store'
 
 export function Chat() {
   const { activeFilePeer } = useWaste()
+  const [sidebarOpen, setSidebarOpen] = useState(false)
   return (
-    <div className={`chat-layout${activeFilePeer ? ' has-file-browser' : ''}`}>
-      <Sidebar />
-      <MessagePane />
+    <div className={`chat-layout${activeFilePeer ? ' has-file-browser' : ''}${sidebarOpen ? ' sidebar-open' : ''}`}>
+      <Sidebar onClose={() => setSidebarOpen(false)} />
+      <MessagePane onMenuClick={() => setSidebarOpen(v => !v)} />
       {activeFilePeer && <FileBrowser />}
     </div>
   )