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/Repositories/IUserRepository.cs

@@ -7,8 +7,10 @@ public interface IUserRepository
     ValueTask<NexusUser?> GetByIdAsync(Guid userId, CancellationToken ct = default);
     Task<NexusUser?> GetByEmailAsync(string normalizedEmail, CancellationToken ct = default);
     Task<bool> AnyUsersAsync(CancellationToken ct = default);
+    Task<List<NexusUser>> GetAllAsync(CancellationToken ct = default);
     Task<NexusUser> AddAsync(NexusUser user, CancellationToken ct = default);
     Task UpdateAsync(NexusUser user, CancellationToken ct = default);
+    Task DeleteAsync(NexusUser user, CancellationToken ct = default);
 
     Task<RefreshToken?> GetRefreshTokenByHashAsync(string tokenHash, CancellationToken ct = default);
     Task<List<RefreshToken>> GetActiveTokensByFamilyAsync(Guid familyId, CancellationToken ct = default);

backend/Repositories/UserRepository.cs

@@ -11,6 +11,9 @@ public sealed class UserRepository(NexusDbContext db) : IUserRepository
     public Task<NexusUser?> GetByEmailAsync(string normalizedEmail, CancellationToken ct = default)
         => db.Users.FirstOrDefaultAsync(u => u.NormalizedEmail == normalizedEmail, ct);
 
+    public Task<List<NexusUser>> GetAllAsync(CancellationToken ct = default)
+        => db.Users.OrderBy(u => u.CreatedAt).ToListAsync(ct);
+
     public Task<bool> AnyUsersAsync(CancellationToken ct = default)
         => db.Users.AnyAsync(ct);
 
@@ -24,6 +27,17 @@ public sealed class UserRepository(NexusDbContext db) : IUserRepository
     public Task UpdateAsync(NexusUser user, CancellationToken ct = default)
         => db.SaveChangesAsync(ct);
 
+    public async Task DeleteAsync(NexusUser user, CancellationToken ct = default)
+    {
+        // Remove refresh tokens first
+        var tokens = await db.RefreshTokens
+            .Where(r => r.UserId == user.Id)
+            .ToListAsync(ct);
+        db.RefreshTokens.RemoveRange(tokens);
+        db.Users.Remove(user);
+        await db.SaveChangesAsync(ct);
+    }
+
     public Task<RefreshToken?> GetRefreshTokenByHashAsync(string tokenHash, CancellationToken ct = default)
         => db.RefreshTokens
             .Include(r => r.User)