Implement code review fixes and improvements

- Move admin credentials from hardcoded values to configuration
- Add rate limiting (5/min) to login endpoint for brute force protection
- Extract CleanJsonResponse to shared JsonResponseHelper class
- Add DateHelpers.MonthsBetween utility and consolidate date calculations
- Update PdfReportService to use ScoreThresholds constants
- Remove 5 unused shared components (EmploymentTable, FlagsList, etc.)
- Clean up unused CSS from MainLayout.razor.css
- Create IPdfReportService interface for better testability
- Add authentication requirement to Hangfire dashboard in development
- Seal EducationVerifierService class

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

src/TrueCV.Application/Helpers/DateHelpers.cs

@@ -2,6 +2,22 @@ namespace TrueCV.Application.Helpers;
 
 public static class DateHelpers
 {
+    /// <summary>
+    /// Calculates the number of months between two dates.
+    /// </summary>
+    public static int MonthsBetween(DateOnly start, DateOnly end)
+    {
+        return ((end.Year - start.Year) * 12) + (end.Month - start.Month);
+    }
+
+    /// <summary>
+    /// Calculates the number of months between two dates.
+    /// </summary>
+    public static int MonthsBetween(DateTime start, DateTime end)
+    {
+        return ((end.Year - start.Year) * 12) + (end.Month - start.Month);
+    }
+
     public static DateOnly? ParseDate(string? dateString)
     {
         if (string.IsNullOrWhiteSpace(dateString))