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/stores/layout.ts

@@ -1,11 +1,17 @@
 import { create } from 'zustand';
 
+export type MobileView = 'sidebar' | 'chat' | 'members';
+
 interface LayoutState {
   isDM: boolean;
   setDM: (value: boolean) => void;
+  mobileView: MobileView;
+  setMobileView: (view: MobileView) => void;
 }
 
 export const useLayoutStore = create<LayoutState>((set) => ({
   isDM: false,
   setDM: (value) => set({ isDM: value }),
+  mobileView: 'chat',
+  setMobileView: (view) => set({ mobileView: view }),
 }));