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/Pages/TaxFilingSchedules.razor

@@ -130,7 +130,7 @@ else
                 </div>
                 <MudForm @ref="form">
                     <MudSelect T="int?"
-                               @bind-Value="scheduleForm.ClientId"
+                               @bind-value="scheduleForm.ClientId"
                                Label="고객"
                                Required="true"
                                Variant="Variant.Outlined"
@@ -143,9 +143,9 @@ else
                             <MudSelectItem Value="@((int?)client.Id)">@GetClientDisplayName(client)</MudSelectItem>
                         }
                     </MudSelect>
-                    <CommonCodeSelect @bind-Value="scheduleForm.FilingType" Group="FILING_TYPE" Label="신고 유형" Class="mb-3" Required="true" />
+                    <CommonCodeSelect @bind-value="scheduleForm.FilingType" Group="FILING_TYPE" Label="신고 유형" Class="mb-3" Required="true" />
                     <MudDatePicker @bind-Date="scheduleForm.DueDate" Label="마감일" Variant="Variant.Outlined" FullWidth="@true" Class="mb-3" Required="true" />
-                    <MudNumericField T="int" @bind-Value="scheduleForm.FilingYear" Label="신고연도" Variant="Variant.Outlined" FullWidth="@true" Class="mb-4" Required="true" />
+                    <MudNumericField T="int" @bind-value="scheduleForm.FilingYear" Label="신고연도" Variant="Variant.Outlined" FullWidth="@true" Class="mb-4" Required="true" />
                     
                     <div class="d-flex justify-end gap-2">
                         @if (isEditMode && selectedSchedule?.Status != "completed")