Restructures project with feature-based organization

Refactors project structure to support modular, feature-driven development

Introduces comprehensive language localization support
Adds menu management with role-based access control
Implements dynamic sidebar and theme switching capabilities

Enhances project scalability and maintainability

app/Features/Menu/Models/MenuGroup.cs

@@ -0,0 +1,12 @@
+namespace CalendarServer.Features.Menu.Models;
+
+/// <summary>
+/// Represents a group of menu items (e.g., "Dashboard", "Data", "System").
+/// </summary>
+public class MenuGroup
+{
+    public required string Id { get; set; }
+    public required string Label { get; set; }
+    public int SortOrder { get; set; }
+    public List<MenuItem> Items { get; set; } = new();
+}

app/Features/Menu/Models/MenuItem.cs

@@ -0,0 +1,15 @@
+namespace CalendarServer.Features.Menu.Models;
+
+/// <summary>
+/// Represents a single menu item in the sidebar.
+/// </summary>
+public class MenuItem
+{
+    public required string Id { get; set; }
+    public required string Label { get; set; }
+    public required string Icon { get; set; }
+    public required string Url { get; set; }
+    public UserRole MinimumRole { get; set; } = UserRole.Staff;
+    public int SortOrder { get; set; }
+    public bool IsActive { get; set; }
+}

app/Features/Menu/Models/UserRole.cs

@@ -0,0 +1,11 @@
+namespace CalendarServer.Features.Menu.Models;
+
+/// <summary>
+/// User roles for menu visibility. Higher value = more access.
+/// </summary>
+public enum UserRole
+{
+    Staff = 0,
+    Manager = 1,
+    Admin = 2
+}