feat: DM infrastructure overhaul + volume sliders + LiveKit dynamic URL

- Add DM edit/delete endpoints (REST + WebSocket)
- Add DM typing indicators with real-time broadcast
- Fix volume sliders to control LiveKit mic/speaker
- Fix Send Message button on user profile popout
- Add New DM modal with user search
- Fix message pagination (SQL cursor instead of in-memory)
- Guard optional attachments on DM messages
- Dynamic LiveKit URL from request Host header
- DM sidebar auto-sorts by most recent message

packages/web/src/components/layout/AppLayout.tsx

@@ -12,6 +12,7 @@ import { CreateChannelModal } from '../modals/CreateChannel';
 import { InviteModal } from '../modals/InviteModal';
 import { UserSettingsModal } from '../modals/UserSettings';
 import { ServerSettingsModal } from '../modals/ServerSettings';
+import { NewDmModal } from '../modals/NewDmModal';
 import { UserProfilePopout } from '../ui/UserProfilePopout';
 import { useAuth } from '../../hooks/useAuth';
 import { useWebSocket } from '../../hooks/useWebSocket';
@@ -145,6 +146,7 @@ export function AppLayout() {
       <InviteModal />
       <UserSettingsModal />
       <ServerSettingsModal />
+      <NewDmModal />
       <ImagePreview />
 
       {/* User Profile Popout */}