Add a user guide, reachable in-app from Help in the account menu

USER_GUIDE.md at the repo root is the single source of truth --
frontend/public/USER_GUIDE.md symlinks to it so the same file is both
readable directly in the repo and served by the app, rendered on a new
/help page reusing the existing markdown renderer. Scoped to regular
member features (messaging, rooms, attachments, notifications,
profile); room admin/site admin features are intentionally left out.

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

frontend/src/App.tsx

@@ -11,6 +11,7 @@ import { ForgotPasswordPage } from './pages/ForgotPasswordPage'
 import { ResetPasswordPage } from './pages/ResetPasswordPage'
 import { ChatShellPage } from './pages/ChatShellPage'
 import { AdminPage } from './pages/AdminPage'
+import { HelpPage } from './pages/HelpPage'
 
 function AppRoutes() {
   const routes = (
@@ -43,6 +44,14 @@ function AppRoutes() {
           </AdminRoute>
         }
       />
+      <Route
+        path="/help"
+        element={
+          <ProtectedRoute>
+            <HelpPage />
+          </ProtectedRoute>
+        }
+      />
       <Route path="*" element={<Navigate to="/rooms" replace />} />
     </Routes>
   )