UI: Add dark theme using Bootstrap 5 dark mode

- Enable data-bs-theme="dark" on html element
- Update navbar to dark variant
- Replace text-dark links with text-body (theme-aware)
- Replace bg-white/bg-light with bg-body/bg-body-secondary
- Update sticky table headers to dark theme
- Fix Chart.js legend and axis colors for dark backgrounds
- Adjust focus ring color for better visibility on dark

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

MoneyMap/wwwroot/css/site.css

@@ -9,7 +9,7 @@ html {
 }
 
 .btn:focus, .btn:active:focus, .btn-link.nav-link:focus, .form-control:focus, .form-check-input:focus {
-  box-shadow: 0 0 0 0.1rem white, 0 0 0 0.25rem #258cfb;
+  box-shadow: 0 0 0 0.25rem rgba(37, 140, 251, 0.5);
 }
 
 html {