feat: implement multi-tenant company management system

Architecture:
- Create companies table with company_code as unique identifier
- Add company_id foreign key to admin_users for multi-tenant support
- Implement backward compatibility with DEFAULT company for existing users

Core Components:
- Company entity with full CRUD operations
- ICompanyRepository interface following Repository pattern
- CompanyRepository with Dapper implementation
- CompanyService with business logic and validation
- CompanyController with REST API endpoints

Admin UI:
- CompanyForm reusable component (Create/Edit pattern)
- CompanyList.razor with pagination and company overview
- CompanyCreate.razor for registering new companies
- CompanyEdit.razor for managing existing companies with delete
- All pages follow admin-page-hero pattern for consistency

SOLID Principles:
- Single Responsibility: Each component has one reason to change
- Open/Closed: Extensible without modifying existing code
- Interface Segregation: Clean repository and service contracts
- Dependency Inversion: All layers depend on abstractions

Database Migration (V014):
- Creates companies table with active/inactive status
- Assigns existing admin users to DEFAULT company
- Provides foundation for role-based access control

Future Enhancement:
- Admin users can belong to specific companies
- Data filtering based on company_id (multi-tenant isolation)
- Company-based permission model

TaxBaik.Domain/Entities/Company.cs

@@ -0,0 +1,15 @@
+namespace TaxBaik.Domain.Entities;
+
+public class Company
+{
+    public int Id { get; set; }
+    public string CompanyCode { get; set; } = "";
+    public string CompanyName { get; set; } = "";
+    public string? ContactPerson { get; set; }
+    public string? Phone { get; set; }
+    public string? Email { get; set; }
+    public string? Memo { get; set; }
+    public bool IsActive { get; set; } = true;
+    public DateTime CreatedAt { get; set; }
+    public DateTime UpdatedAt { get; set; }
+}