Add missing login page and fix auth loop

- /login route did not exist, so /admin redirected to /api/login -> /login -> 404
- /api/auth/user still ran the Replit-era deterministic UUID hash on the session
  id, so it never found the user; now reads the session directly
- admin logout posts to /api/auth/logout and returns to /login

client/src/App.tsx

@@ -13,6 +13,7 @@ import LivePage from "@/pages/LivePage";
 import PlayerPage from "@/pages/PlayerPage";
 import KuenstlerPage from "@/pages/KuenstlerPage";
 import AdminPage from "@/pages/admin";
+import LoginPage from "@/pages/login";
 import PrivacyPolicy from "@/pages/PrivacyPolicy";
 import TermsOfService from "@/pages/TermsOfService";
 import Impressum from "@/pages/Impressum";
@@ -29,6 +30,7 @@ function Router() {
       <Route path="/live" component={LivePage} />
       <Route path="/player" component={PlayerPage} />
       <Route path="/kuenstler" component={KuenstlerPage} />
+      <Route path="/login" component={LoginPage} />
       <Route path="/admin" component={AdminPage} />
       <Route path="/privacy" component={PrivacyPolicy} />
       <Route path="/terms" component={TermsOfService} />