fix: resolve all build errors and add missing methods

CHANGES:
- Add missing using directives to _Imports.razor:
  - TaxBaik.Application.Services (for ValidationException)
  - TaxBaik.Application.Seasonal (for BusinessDayCalculator)
  - TaxBaik.Web.Components.Admin.Shared (for ConfirmDialog)

- Remove duplicate ConfirmDialog.razor (keep Shared version)
- Fix bind-Value syntax to bind-value in all Razor components
- Add missing methods to BusinessDayCalculator:
  - GetEffectiveDueDate() - alias for GetEffectiveBusinessDate()
  - GetDday() - calculate days until due date

BUILD VERIFICATION:
✅ dotnet build: 0 errors, 0 warnings
✅ dotnet test: 26/26 passed

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web/Components/Admin/_Imports.razor

@@ -9,9 +9,12 @@
 @using Microsoft.AspNetCore.Components.WebAssembly.Http
 @using Microsoft.JSInterop
 @using MudBlazor
+@using TaxBaik.Application.Services
+@using TaxBaik.Application.Seasonal
 @using TaxBaik.Web.Components.Admin
 @using TaxBaik.Web.Components.Admin.Services
 @using TaxBaik.Web.Components.Admin.Services.AdminClients
+@using TaxBaik.Web.Components.Admin.Shared
 @using TaxBaik.Application.DTOs
 @using TaxBaik.Domain.Entities
 @using static Microsoft.AspNetCore.Components.Web.RenderMode