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/src/views/Dashboard/FlowBoard.vue

@@ -18,7 +18,6 @@ import { useAgentStore } from '../../stores/agents'
 import { useChatStore } from '../../stores/chat'
 import { useDashboardStore } from '../../stores/dashboard'
 import { useTaskStore } from '../../stores/tasks'
-import { useLiveSyncStore } from '../../stores/liveSync'
 import AlertBar from '../../components/dashboard/v2/AlertBar.vue'
 import FlowCanvas from '../../components/dashboard/v2/FlowCanvas.vue'
 import IrisChat from '../../components/dashboard/v2/IrisChat.vue'
@@ -31,7 +30,6 @@ const agentStore = useAgentStore()
 const chatStore = useChatStore()
 const dashboardStore = useDashboardStore()
 const taskStore = useTaskStore()
-const liveSyncStore = useLiveSyncStore()
 const router = useRouter()
 
 const {
@@ -70,7 +68,6 @@ onMounted(() => {
   dashboardStore.startPolling()
   taskStore.startPolling()
   taskStore.startBoardPolling()
-  liveSyncStore.connect()
 })
 
 onUnmounted(() => {
@@ -79,7 +76,6 @@ onUnmounted(() => {
   dashboardStore.stopPolling()
   taskStore.stopPolling()
   taskStore.stopBoardPolling()
-  liveSyncStore.disconnect()
 })
 </script>