Rebuild frontend from the Claude Design handoff, DarkSingularity brand

Replaces the Phase 1 placeholder UI with a single persistent app shell (top
bar + sidebar + chat pane, 860px responsive breakpoint) matching the
"PWA chat system UI" design handoff: message bubbles with consecutive-run
avatar/name grouping, auto-growing composer, room search, and the real
DarkSingularity logo (also used to regenerate the PWA icons).

The handoff didn't cover Phase 2 (private rooms, roles, invites) or
browsing/joining open rooms, so those are added using the same visual
language: a room info panel with role badges, invite-by-username with a
pending-invites list, member management (remove/promote/demote/transfer
ownership), room settings (rename/describe/delete), and separate
browse-rooms/invites-inbox modals. Unread badges, last-message preview, and
the typing indicator are deliberately deferred -- both need new backend
features (read-tracking, a WS typing event) that weren't in scope this pass.

Two small backend additions round out data the new UI needs but the API
didn't expose: MessageRead.username (historic messages had no sender name)
and InviteRead.target_username / MyInviteRead.room_name+invited_by_username
(a recipient's invite list can't otherwise resolve a room they're not in).
Both are additive; 35 backend tests still pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -2,8 +2,7 @@ import { Navigate, Route, Routes } from 'react-router-dom'
 import { AuthProvider } from './context/AuthContext'
 import { ProtectedRoute } from './components/ProtectedRoute'
 import { LoginPage } from './pages/LoginPage'
-import { RoomListPage } from './pages/RoomListPage'
-import { ChatRoomPage } from './pages/ChatRoomPage'
+import { ChatShellPage } from './pages/ChatShellPage'
 
 function App() {
   return (
@@ -14,7 +13,7 @@ function App() {
           path="/rooms"
           element={
             <ProtectedRoute>
-              <RoomListPage />
+              <ChatShellPage />
             </ProtectedRoute>
           }
         />
@@ -22,7 +21,7 @@ function App() {
           path="/rooms/:roomId"
           element={
             <ProtectedRoute>
-              <ChatRoomPage />
+              <ChatShellPage />
             </ProtectedRoute>
           }
         />