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>

backend/Services/TaskService.cs

@@ -431,7 +431,9 @@ public sealed class TaskService(
 
         foreach (var task in all)
         {
-            var dto = MapToDtoWithChildren(task, all, activity);
+            // Ohne verschachtelte Child-DTOs: Children sind als eigene Karten im Board,
+            // die Nested-Duplikate haben die Payload nur verdoppelt (Counts bleiben).
+            var dto = MapToDtoWithChildren(task, all, activity, includeChildren: false);
             switch (task.State.ToLowerInvariant())
             {
                 case "backlog": offen.Add(dto); break;
@@ -513,22 +515,26 @@ public sealed class TaskService(
         return all.Where(e => e.TaskId == taskId).ToList();
     }
 
-    private DashboardTaskDto MapToDtoWithChildren(WorkTask task, IReadOnlyList<WorkTask> allTasks, IEnumerable<ActivityEvent> activity)
+    private DashboardTaskDto MapToDtoWithChildren(WorkTask task, IReadOnlyList<WorkTask> allTasks, IEnumerable<ActivityEvent> activity, bool includeChildren = true)
     {
         var childTasks = allTasks.Where(t => t.ParentTaskId == task.Id)
             .OrderByDescending(t => t.UpdatedAt)
             .ToList();
 
-        var childDtos = childTasks.Select(child => MapToDtoWithActivity(child, activity, allTasks)).ToList();
+        // includeChildren=false (Board/SSE-Snapshot): Children erscheinen dort ohnehin
+        // als eigene Karten — verschachtelte Child-DTOs verdoppeln nur die Payload.
+        var childDtos = includeChildren
+            ? childTasks.Select(child => MapToDtoWithActivity(child, activity, allTasks)).ToList()
+            : null;
         var openChildTaskCount = childTasks.Count(child => !string.Equals(child.State, "Done", StringComparison.OrdinalIgnoreCase));
 
         var dto = MapToDtoWithActivity(task, activity, allTasks);
         return dto with
         {
             ChildTasks = childDtos,
-            ChildTaskCount = childDtos.Count,
+            ChildTaskCount = childTasks.Count,
             OpenChildTaskCount = openChildTaskCount,
-            HasVisibleDelegation = dto.ParentTaskId.HasValue || childDtos.Count > 0 || dto.IsAgentTask
+            HasVisibleDelegation = dto.ParentTaskId.HasValue || childTasks.Count > 0 || dto.IsAgentTask
         };
     }