feat(welcome): add welcome page for new users after first login

Shows six feature cards and two next steps (Steam, import). The dashboard
redirects to it while Auth settings.welcomeSeenAt is null; "Let's go" and
"Skip" set the stamp. Settings links to it to reopen later.

src/App.jsx

@@ -11,6 +11,8 @@ import SettingsPage from './pages/SettingsPage.jsx';
 import AdminHintsPage from './pages/AdminHintsPage.jsx';
 import AdminNewsPage from './pages/AdminNewsPage.jsx';
 import WhatsNewPage from './pages/WhatsNewPage.jsx';
+import WelcomePage from './pages/WelcomePage.jsx';
+import WelcomeGate from './components/WelcomeGate.jsx';
 import AdminUsersPage from './pages/AdminUsersPage.jsx';
 import GameLibrary from './pages/GameLibrary.jsx';
 import GameDetail from './pages/GameDetail.jsx';
@@ -65,7 +67,7 @@ function App() {
                         <Route path="/login" element={<LoginPage/>}/>
                         <Route path="/register" element={<RegistrationPage/>}/>
                         <Route element={<RequireSession/>}>
-                            <Route path="/dashboard" element={<Dashboard/>}/>
+                            <Route path="/dashboard" element={<WelcomeGate><Dashboard/></WelcomeGate>}/>
                             <Route path="/library" element={<GameLibrary/>}/>
                             <Route path="/library/:appid" element={<GameDetail/>}/>
                             <Route path="/wishlist" element={<WishlistPage/>}/>
@@ -74,6 +76,7 @@ function App() {
                             <Route path="/import" element={<ImportPage/>}/>
                             <Route path="/settings" element={<SettingsPage/>}/>
                             <Route path="/whats-new" element={<WhatsNewPage/>}/>
+                            <Route path="/welcome" element={<WelcomePage/>}/>
                             <Route path="/admin/users" element={<AdminUsersPage/>}/>
                             <Route path="/admin/hints" element={<AdminHintsPage/>}/>
                             <Route path="/admin/news" element={<AdminNewsPage/>}/>