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.Web/Components/Admin/Pages/Companies/CompanyCreate.razor

@@ -0,0 +1,51 @@
+@page "/admin/companies/create"
+@attribute [Authorize]
+@using TaxBaik.Web.Components.Admin.Forms
+@inject IApiClient ApiClient
+@inject NavigationManager Navigation
+@inject ISnackbar Snackbar
+
+<PageTitle>고객사 등록</PageTitle>
+
+<section class="admin-page-hero">
+    <div>
+        <MudText Typo="Typo.caption" Class="admin-eyebrow">Settings</MudText>
+        <MudText Typo="Typo.h4" Class="admin-page-title">새 고객사 등록</MudText>
+        <MudText Typo="Typo.body2" Class="admin-page-subtitle">새로운 고객사를 추가합니다.</MudText>
+    </div>
+    <MudButton Variant="Variant.Outlined" StartIcon="@Icons.Material.Filled.Close" @onclick="GoBack">취소</MudButton>
+</section>
+
+<MudPaper Class="pa-4 mt-4" Elevation="1">
+    <CompanyForm ButtonText="등록" OnSubmit="HandleCreate" OnCancel="GoBack" />
+</MudPaper>
+
+@code {
+    private void GoBack()
+    {
+        Navigation.NavigateTo("/taxbaik/admin/companies");
+    }
+
+    private async Task HandleCreate(CompanyForm.CompanyFormModel model)
+    {
+        try
+        {
+            await ApiClient.PostAsync<object>("company", new
+            {
+                companyCode = model.CompanyCode,
+                companyName = model.CompanyName,
+                contactPerson = model.ContactPerson,
+                phone = model.Phone,
+                email = model.Email,
+                memo = model.Memo
+            });
+
+            Snackbar.Add("고객사가 등록되었습니다.", Severity.Success);
+            Navigation.NavigateTo("/taxbaik/admin/companies");
+        }
+        catch (Exception ex)
+        {
+            Snackbar.Add($"등록 실패: {ex.Message}", Severity.Error);
+        }
+    }
+}