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/App.css

@@ -164,3 +164,64 @@ details summary { color: var(--muted); font-size: 12px; cursor: pointer; }
 .file-entry-icon { font-size: 12px; flex-shrink: 0; }
 .history-divider { display: flex; align-items: center; gap: 8px; margin: 10px 0 6px; color: var(--muted); font-size: 11px; }
 .history-divider::before, .history-divider::after { content: ''; flex: 1; height: 1px; background: var(--border); }
+
+/* ── mobile hamburger / close buttons ── */
+.menu-btn-mobile { display: none; background: none; color: var(--muted); font-size: 18px; padding: 0 8px 0 0; line-height: 1; }
+.menu-btn-mobile:hover { color: var(--text); background: none; }
+.sidebar-close-mobile { display: none; background: none; color: var(--muted); font-size: 14px; padding: 2px 4px; }
+.sidebar-close-mobile:hover { color: var(--text); background: none; }
+
+/* ── responsive layout ── */
+@media (max-width: 600px) {
+  :root { --sidebar-w: 80vw; }
+
+  .chat-layout {
+    grid-template-columns: 1fr;
+    position: relative;
+  }
+
+  /* sidebar slides in over the top */
+  .sidebar {
+    position: fixed;
+    top: 0; left: 0;
+    width: var(--sidebar-w);
+    height: 100%;
+    z-index: 100;
+    transform: translateX(-100%);
+    transition: transform 0.22s ease;
+    box-shadow: 4px 0 24px rgba(0,0,0,0.5);
+  }
+  .chat-layout.sidebar-open .sidebar {
+    transform: translateX(0);
+  }
+
+  /* dim overlay behind open sidebar */
+  .chat-layout.sidebar-open::before {
+    content: '';
+    position: fixed;
+    inset: 0;
+    background: rgba(0,0,0,0.45);
+    z-index: 99;
+  }
+
+  .menu-btn-mobile { display: inline-block; }
+  .sidebar-close-mobile { display: inline-block; }
+
+  /* message pane fills full width */
+  .chat-layout > .message-pane { grid-column: 1; }
+
+  /* file browser stacks below on mobile */
+  .chat-layout.has-file-browser { grid-template-columns: 1fr; }
+  .chat-layout.has-file-browser > .file-browser { border-left: none; border-top: 1px solid var(--border); max-height: 40vh; overflow-y: auto; }
+
+  /* slightly larger tap targets */
+  .sidebar-item { padding: 8px 12px; font-size: 14px; }
+  .peer-row { padding: 6px 12px; }
+  .peer-row-actions { opacity: 1; }
+  .peer-action { font-size: 18px; padding: 2px 6px; }
+
+  /* message layout: stack alias above text on very narrow screens */
+  .message { flex-wrap: wrap; }
+  .message-ts { width: auto; min-width: 56px; }
+  .message-alias { width: auto; }
+}

web/src/components/MessagePane.tsx

@@ -12,7 +12,7 @@ function formatTs(ts: number): string {
   return d.toLocaleDateString([], { month: 'short', day: 'numeric' }) + ' ' + time
 }
 
-export function MessagePane() {
+export function MessagePane({ onMenuClick }: { onMenuClick: () => void }) {
   const { messages, historyCutoff, activeRoom, activeNetworkId, localPeer, connectedPeers, knownPeers, send } = useWaste()
   const [draft, setDraft] = useState('')
   const bottomRef = useRef<HTMLDivElement>(null)
@@ -64,7 +64,10 @@ export function MessagePane() {
 
   return (
     <main className="message-pane">
-      <div className="message-pane-header">{roomLabel}</div>
+      <div className="message-pane-header">
+        <button className="menu-btn-mobile" onClick={onMenuClick} aria-label="Menu">☰</button>
+        {roomLabel}
+      </div>
 
       <div className="messages">
         {dividerIdx === 0 && (

web/src/components/Sidebar.tsx

@@ -26,7 +26,7 @@ function formatTs(ts: number | undefined): string {
   return new Date(ts).toLocaleTimeString([], { hour: '2-digit', minute: '2-digit', second: '2-digit' })
 }
 
-export function Sidebar() {
+export function Sidebar({ onClose }: { onClose: () => void }) {
   const {
     localPeer, masterId, masterAlias,
     networks, activeNetworkId, activeRoom,
@@ -118,6 +118,7 @@ export function Sidebar() {
           <span className="peer-id">{displayId.slice(0, 16).replace(/(.{4})/g, '$1 ').trim()}</span>
         </div>
         <button className="sidebar-logout" onClick={handleLogout} title="Leave network">⏻</button>
+        <button className="sidebar-close-mobile" onClick={onClose} title="Close">✕</button>
       </div>
 
       <div className="sidebar-section">
@@ -134,7 +135,7 @@ export function Sidebar() {
           <button
             key={n.network_id}
             className={`sidebar-item ${n.network_id === activeNetworkId ? 'active' : ''}`}
-            onClick={() => setActiveNetwork(n.network_id)}
+            onClick={() => { setActiveNetwork(n.network_id); onClose() }}
           >
             {n.network_name}
           </button>
@@ -173,7 +174,7 @@ export function Sidebar() {
           <button
             key={r}
             className={`sidebar-item ${r === activeRoom ? 'active' : ''}`}
-            onClick={() => setActiveRoom(r)}
+            onClick={() => { setActiveRoom(r); onClose() }}
           >
             {r.startsWith('dm:') ? `@ ${r.slice(3, 11)}…` : `# ${r}`}
           </button>

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>
   )