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/stores/liveSync.ts

@@ -50,9 +50,11 @@ export const useLiveSyncStore = defineStore('liveSync', {
     error: null as string | null,
     controller: null as AbortController | null,
     reconnectTimer: null as ReturnType<typeof setTimeout> | null,
+    watchdogTimer: null as ReturnType<typeof setInterval> | null,
     mode: 'polling' as 'polling' | 'live',
     lastSequence: 0,
     reconnectAttempts: 0,
+    forUser: 'bao',
   }),
 
   getters: {
@@ -73,7 +75,9 @@ export const useLiveSyncStore = defineStore('liveSync', {
       if (this.connecting || this.connected) return
       this.connecting = true
       this.error = null
+      this.forUser = forUser
       this.controller = new AbortController()
+      this.startWatchdog()
 
       const taskStore = useTaskStore()
       const notificationStore = useNotificationStore()
@@ -113,19 +117,51 @@ export const useLiveSyncStore = defineStore('liveSync', {
         }, { forUser, signal: this.controller.signal, afterSequence: this.lastSequence || null })
 
         await stream.closed
+        // Stream „sauber" beendet (Proxy-Timeout, Server-Neustart, Netzwechsel):
+        // muss genauso wie ein Fehler behandelt werden — sonst bleibt connected=true
+        // hängen, Polling ist gestoppt und die Seite erhält nie wieder Updates.
       } catch (error) {
-        if (this.controller?.signal.aborted) return
-        console.warn('[liveSync] stream failed, falling back to polling', error)
-        this.error = 'Live updates unavailable'
-        this.connected = false
-        this.mode = 'polling'
-        taskStore.startBoardPolling()
-        this.scheduleReconnect(forUser)
+        if (!this.controller?.signal.aborted) {
+          console.warn('[liveSync] stream failed', error)
+          this.error = 'Live updates unavailable'
+        }
       } finally {
         this.connecting = false
-        if (!this.controller?.signal.aborted && !this.connected) {
-          this.mode = 'polling'
+      }
+
+      if (this.controller?.signal.aborted) return
+
+      this.connected = false
+      this.mode = 'polling'
+      taskStore.startBoardPolling()
+      this.scheduleReconnect(forUser)
+    },
+
+    /** Erzwingt einen frischen Stream (Watchdog / visibilitychange / online). */
+    reconnectNow() {
+      const forUser = this.forUser
+      this.disconnect()
+      this.connect(forUser)
+    },
+
+    startWatchdog() {
+      if (this.watchdogTimer) return
+      this.watchdogTimer = setInterval(() => {
+        if (!this.connected || !this.lastEventAt) return
+        // Heartbeat kommt alle 20s — >65s Stille heißt: Verbindung ist tot,
+        // auch wenn der Browser den fetch-Stream noch für offen hält.
+        const silentMs = Date.now() - new Date(this.lastEventAt).getTime()
+        if (silentMs > 65000) {
+          console.warn('[liveSync] heartbeat timeout, reconnecting')
+          this.reconnectNow()
         }
+      }, 15000)
+    },
+
+    stopWatchdog() {
+      if (this.watchdogTimer) {
+        clearInterval(this.watchdogTimer)
+        this.watchdogTimer = null
       }
     },
 
@@ -148,6 +184,7 @@ export const useLiveSyncStore = defineStore('liveSync', {
     },
 
     disconnect() {
+      this.stopWatchdog()
       this.controller?.abort()
       this.controller = null
       this.connected = false
@@ -161,7 +198,9 @@ export const useLiveSyncStore = defineStore('liveSync', {
 
     scheduleReconnect(forUser = 'bao') {
       if (this.reconnectTimer) return
-      const delay = Math.min(30000, 5000 * Math.max(1, this.reconnectAttempts + 1))
+      // Erster Retry schnell (1s) — der häufigste Fall ist ein Proxy-/Deploy-Cut,
+      // danach sanft hochstaffeln bis 30s.
+      const delay = this.reconnectAttempts === 0 ? 1000 : Math.min(30000, 5000 * this.reconnectAttempts)
       this.reconnectAttempts += 1
       this.reconnectTimer = setTimeout(() => {
         this.reconnectTimer = null