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.Application/Seasonal/BusinessDayCalculator.cs

@@ -68,6 +68,14 @@ public static class BusinessDayCalculator
         return effectiveDate.DayNumber - referenceDate.DayNumber;
     }
 
+    public static DateOnly GetEffectiveDueDate(DateOnly date) => GetEffectiveBusinessDate(date);
+
+    public static int GetDday(DateOnly dueDate)
+    {
+        var today = DateOnly.FromDateTime(DateTime.Now);
+        return dueDate.DayNumber - today.DayNumber;
+    }
+
     private static bool IsBusinessDay(DateOnly date)
         => date.DayOfWeek is not DayOfWeek.Saturday and not DayOfWeek.Sunday
            && !HolidayDates.Contains(date);