feat: mobile-responsive layout

- Mobile (<md): single-panel view, sidebar/chat/members as overlays
- Hamburger button toggles sidebar overlay (ServerBar + ChannelList)
- Members button toggles MemberList overlay
- ← back button in ChatArea and DMChat headers returns to sidebar
- Auto-switch to chat view when channel or DM is selected
- Tap empty area to dismiss overlays
- Desktop layout unchanged (all panels side by side)
- Status/settings buttons hidden on small screens to save space

web/src/components/ChatArea.tsx

@@ -22,6 +22,7 @@ import { FeatureRequestsPanel } from "./FeatureRequestsPanel.tsx";
 import { ReactionBar } from "./ReactionBar.tsx";
 import { EmojiPicker } from "./EmojiPicker.tsx";
 import { ReplyBar } from "./ReplyBar.tsx";
+import { useLayoutStore } from "../stores/layout.ts";
 import { ForumView } from "./ForumView.tsx";
 import { CalendarView } from "./CalendarView.tsx";
 import { DocsView } from "./DocsView.tsx";
@@ -629,22 +630,31 @@ export function ChatArea() {
 
   return (
     <div className="flex flex-col h-full bg-gb-bg">
-      <div className="terminal-border border-t-0 border-x-0 px-3 py-2 text-gb-fg-s flex items-center justify-between">
-        <span>
-          {activeChannel
-            ? activeChannel.type === 'forum'
-              ? `■ ${activeChannel.name}`
-              : activeChannel.type === 'calendar'
-                ? `○ ${activeChannel.name}`
-                : activeChannel.type === 'docs'
-                  ? `☰ ${activeChannel.name}`
-                  : activeChannel.type === 'list'
-                    ? `☑ ${activeChannel.name}`
-                    : `# ${activeChannel.name}`
-            : activeChannelId
-              ? `#${activeChannelId}`
-              : "[NO CHANNEL SELECTED]"}
-        </span>
+      <div className="terminal-border border-t-0 border-x-0 px-2 md:px-3 py-2 text-gb-fg-s flex items-center justify-between gap-2">
+        <div className="flex items-center gap-2 min-w-0">
+          <button
+            type="button"
+            onClick={() => useLayoutStore.getState().setMobileView('sidebar')}
+            className="md:hidden terminal-button text-xs px-2 py-0.5 shrink-0"
+          >
+            ←
+          </button>
+          <span className="truncate">
+            {activeChannel
+              ? activeChannel.type === 'forum'
+                ? `■ ${activeChannel.name}`
+                : activeChannel.type === 'calendar'
+                  ? `○ ${activeChannel.name}`
+                  : activeChannel.type === 'docs'
+                    ? `☰ ${activeChannel.name}`
+                    : activeChannel.type === 'list'
+                      ? `☑ ${activeChannel.name}`
+                      : `# ${activeChannel.name}`
+              : activeChannelId
+                ? `#${activeChannelId}`
+                : "[NO CHANNEL SELECTED]"}
+          </span>
+        </div>
         {activeChannelId && activeChannel?.type === 'text' && (
           <div className="flex items-center gap-2">
             <button