feat: Multi-User/Admin usermanagement + Galaxy Login/Settings + Task detail improvements

- Backend: NEW AdminController with user CRUD (GET/POST/DELETE /api/v1/admin/users)
- Backend: NEW GET /api/dashboard/tasks/{id} single task endpoint
- Backend: NEW POST /api/dashboard/tasks/{id}/activity comment endpoint
- Backend: IUserRepository + UserRepository extended with GetAllAsync, DeleteAsync
- Backend: Admin DTOs (AdminUserInfo, AdminCreateUserRequest, AdminUpdateRoleRequest)
- Frontend: NEW TaskDetailView.vue — URL-based (/tasks/:id) Galaxy-themed task detail
  with subtask create/edit/delete, activity with comments, property sidebar
- Frontend: LoginView.vue — полностью Galaxy theme redesign with GalaxyBackground,
  glass-morphism card, password toggle, consistent brand
- Frontend: SettingsView.vue — Galaxy theme redesign with glass cards,
  admin user management section (create/list users, visible only to owner role)
- Frontend: TaskBoardView.vue — added "Full View" link to URL-based detail page
- Frontend: Router — added /tasks/:id route for TaskDetailView
- Frontend: App.vue — added TaskDetail to standaloneViews whitelist
- Frontend: tasks store — stable

Auth: Admin creates accounts, users log in with existing /api/v1/auth/login.
Login/Settings deliver visible Galaxy-consistent design with nexus-tokens.css tokens.

backend/Models/Dashboard.cs

@@ -134,10 +134,6 @@ public sealed record MoveTaskRequest(
     string State
 );
 
-public sealed record ImportResultDto(
-    int Imported
-);
-
 public sealed record ResetStaleRequest(
     int StaleHours = 2
 );
@@ -146,6 +142,11 @@ public sealed record ResetStaleResponse(
     int ResetCount
 );
 
+public sealed record PostActivityRequest(
+    string Message,
+    string? Type = null
+);
+
 // ── Notification DTOs ──
 
 public sealed record NotificationDto(