feat(admin): live admin dashboard with solo presence tracking

Solo games run entirely in the browser, so the server previously had no
idea who was playing right now (it only saw logins and finished games).
This adds lightweight presence reporting and an admin dashboard.

Presence (server):
- POST /api/presence (logged-in users): heartbeat while a solo game runs,
  stores username, map (validated), difficulty, wave, since/lastSeen per
  user; entries expire automatically after 90s without a heartbeat
- POST /api/presence/stop: explicit removal when the player exits

Admin API (ADMIN_USERS env, comma-separated usernames):
- GET /api/admin/overview: live solo players, multiplayer room summaries
  (code, mode, map, players — the server already tracks rooms), and
  global stats (accounts, rounds, crystals in circulation)
- GET /api/admin/users?limit=100: user list with stats, newest login first
- publicUser now carries an admin flag; non-admins get 403

Admin UI (src/components/AdminDashboard.vue, served at /admin):
- Login gate for guests/non-admins (guest profiles are detected via
  isLoggedIn, not just user presence)
- KPI cards, live solo table (player, map, difficulty, wave, duration),
  room table, and account table; auto-refresh every 5 seconds
- App.vue renders the dashboard for /admin instead of the game and runs
  a screen watcher that starts/stops the solo presence heartbeat

Config: ADMIN_USERS documented in docker-compose.yml and README.

Tests: 6 new integration checks (admin flag, presence report/stop,
403 guard, overview contents, user list) — 37/37 green, build clean.
Verified end-to-end in the browser: guest gate, admin login, and a live
second player (map/difficulty/wave) appearing in the dashboard.

README.md

@@ -85,6 +85,17 @@ npm run build
 npm run server   # serves dist/ + ws auf :3001
 ```
 
+### 🛡️ Admin-Dashboard (`/admin`)
+
+Live-Übersicht über den Server: wer gerade **Solo spielt** (Karte, Schwierigkeit, Welle — angemeldete Spieler senden dazu alle 20 s einen Heartbeat), offene **Multiplayer-Räume**, Account-**Statistiken** und die **Nutzerliste**. Aktualisiert sich alle 5 s automatisch.
+
+Freischalten über die Umgebungsvariable `ADMIN_USERS` (Komma-getrennte Benutzernamen):
+```yaml
+environment:
+  - ADMIN_USERS=deinaccount
+```
+Dann als eines dieser Konten unter `https://deine-domain/admin` anmelden. Solo-Spiele laufen zwar lokal im Browser, aber angemeldete Spieler melden ihren Spielstatus an den Server; Gäste bleiben unsichtbar.
+
 ---
 
 ## Spielmodi