feat(dashboard): open Player Dashboard in a new tab

The 👥 Dashboard button used to open the player table as a draggable
in-app window, which competed for screen space with the map. It now
opens in a separate browser tab as a fullscreen page so users can put
the dashboard on a second monitor.

How:
- App.tsx branches on ?view=dashboard → renders PlayerDashboardFullPage
  (new file in components/) instead of the default MapLayout.
- SidebarWindowButtons.tsx: 👥 Dashboard onClick now does
  window.open('/?view=dashboard', '_blank', 'noopener'). Label shows
  '↗' so users know it's an external open.
- PlayerDashboardWindow.tsx refactored: extracted the sortable table
  body into a reusable PlayerDashboardContent component. The old window
  shell stays registered in WindowRenderer for backward compat — just
  no longer reachable from the default sidebar.
- map-layout.css: new .ml-dashboard-page rules for fullscreen layout.

Each tab gets its own useLiveData + WebSocket connection (server
already handles multiple browser clients). The new tab inherits the
session cookie from the original tab — no re-login.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,8 +1,28 @@
 import { MapLayout } from './components/map/MapLayout';
+import { PlayerDashboardFullPage } from './components/PlayerDashboardFullPage';
 import { useLiveData } from './hooks/useLiveData';
 import './styles/map-layout.css';
 
+/**
+ * Single SPA entry. Branches on `?view=` query param:
+ *   /?view=dashboard  → fullscreen PlayerDashboardFullPage (new-tab target)
+ *   /                 → default map + sidebar layout
+ *
+ * We don't pull in react-router for one extra view — when a third view
+ * appears, swap this for proper routing.
+ */
 export default function App() {
+  const view = new URLSearchParams(window.location.search).get('view');
+  if (view === 'dashboard') {
+    return <PlayerDashboardFullPage />;
+  }
+  // Default: full app with map + sidebar.
+  return <DefaultApp />;
+}
+
+/** Default map-and-sidebar layout. Split out so the dashboard tab doesn't
+ *  spin up useLiveData twice for the same render. */
+function DefaultApp() {
   const data = useLiveData();
   return <MapLayout data={data} />;
 }