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/TaskBoardView.vue

@@ -17,7 +17,7 @@ import { Plus, X, CalendarDays, Clock3, ExternalLink, Link2, ListChecks, Save, A
 import { useRouter } from 'vue-router'
 import { useAuthStore } from '../stores/auth'
 import { useTaskStore } from '../stores/tasks'
-import { useLiveSyncStore } from '../stores/live-sync'
+import { useLiveSyncStore } from '../stores/liveSync'
 import { TASK_AGENT_LABELS, TASK_AGENT_OPTIONS } from '../constants/agentPool'
 
 type BoardTask = ReturnType<typeof flattenBoard>[number]
@@ -393,7 +393,6 @@ let agentOverviewInterval: ReturnType<typeof setInterval> | null = null
 onMounted(() => {
   taskStore.startBoardPolling()
   taskStore.fetchAgentOverview()
-  liveSyncStore.connect()
   window.addEventListener('keydown', onGlobalKeydown)
   agentOverviewInterval = setInterval(() => taskStore.fetchAgentOverview(), 30000)
 })
@@ -404,7 +403,6 @@ onBeforeUnmount(() => {
 
 onUnmounted(() => {
   taskStore.stopBoardPolling()
-  liveSyncStore.disconnect()
   if (agentOverviewInterval) clearInterval(agentOverviewInterval)
   window.removeEventListener('keydown', onGlobalKeydown)
 })