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.Web/Components/Pages/Dashboard.razor

@@ -8,7 +8,7 @@
 @inject AuthenticationStateProvider AuthenticationStateProvider
 @inject ILogger<Dashboard> Logger
 @inject IJSRuntime JSRuntime
-@inject TrueCV.Web.Services.PdfReportService PdfReportService
+@inject TrueCV.Web.Services.IPdfReportService PdfReportService
 @inject IAuditService AuditService
 
 <PageTitle>Dashboard - TrueCV</PageTitle>

src/TrueCV.Web/Components/Pages/Report.razor

@@ -9,7 +9,7 @@
 @inject ILogger<Report> Logger
 @inject IJSRuntime JSRuntime
 @inject IAuditService AuditService
-@inject PdfReportService PdfReportService
+@inject IPdfReportService PdfReportService
 
 <PageTitle>Verification Report - TrueCV</PageTitle>