feat: unified rail shell, robust live sync, task board performance

Shell:
- One shared NexusLayout for ALL routes (dashboard + pages): compact 68px
  icon rail with hover-expand overlay, replaces both old sidebars + topbars
- Single flat nav source (railNav) — same menu everywhere incl. Settings
- Removed dead shell components (AppSidebar, AppHeader, Topbar, NavGroup,
  NavItem, ModuleView) and dead nav routes; App.vue is now just RouterView

Live updates:
- Fix SSE loop in DashboardController: PeriodicTimer.WaitForNextTickAsync and
  ChannelReader.ReadAsync were re-invoked while pending — every published
  update threw InvalidOperationException and killed ALL live streams
- liveSync store: treat graceful stream close as disconnect (was stuck
  connected=true with polling stopped -> page frozen until manual reload),
  fast first retry, heartbeat watchdog (65s), reconnect on online/visibility
- One app-wide SSE connection owned by the layout instead of per-view
  connect/disconnect churn; removed duplicate live-sync.ts store

Performance:
- Board endpoint: drop nested childTasks duplication (counts stay) — payload
  104KB -> 65KB; SSE snapshots shrink equally
- nginx: gzip for JSON/JS/CSS (board 18KB, bundle 95KB over the wire);
  text/event-stream excluded to keep SSE unbuffered

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

frontend/nginx.conf

@@ -5,6 +5,15 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
+    # Kompression für Bundle + API-JSON (Board-Payload ~100 KB → wenige KB).
+    # text/event-stream bewusst NICHT in gzip_types: gzip würde den SSE-Stream puffern.
+    gzip on;
+    gzip_comp_level 5;
+    gzip_min_length 1024;
+    gzip_vary on;
+    gzip_proxied any;
+    gzip_types application/json application/javascript text/css text/javascript image/svg+xml;
+
     add_header Content-Security-Policy "default-src 'self'; connect-src 'self'; img-src 'self' data:; script-src 'self'; style-src 'self' 'unsafe-inline'; frame-ancestors 'none'; base-uri 'self'; form-action 'self'" always;
     add_header Referrer-Policy "no-referrer" always;
     add_header X-Content-Type-Options "nosniff" always;