Accessibility fixes and form session expiration fix

- Fix color contrast: change #179e83 to #148069 for WCAG AA compliance
- Add ARIA attributes to mobile nav toggle (aria-expanded, aria-controls)
- Implement focus trap on mobile menu with Escape key support
- Add aria-hidden to decorative hero SVG
- Add ARIA validation to contact form (aria-invalid, aria-describedby)
- Fix touch target sizes (notification close button 48x48px)
- Fix form session expiration by relaxing timestamp validation
- Add cache busting (v1.1.0) to JS/CSS files
- Update service worker cache version to force refresh

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

sw.js

@@ -1,10 +1,10 @@
 // UK Data Services - Service Worker for PWA Functionality
 // Version 1.0 - Advanced caching and offline support
 
-const CACHE_NAME = 'ukds-pwa-v1.0.0';
-const STATIC_CACHE = 'ukds-static-v1.0.0';
-const DYNAMIC_CACHE = 'ukds-dynamic-v1.0.0';
-const IMAGE_CACHE = 'ukds-images-v1.0.0';
+const CACHE_NAME = 'ukds-pwa-v1.1.0';
+const STATIC_CACHE = 'ukds-static-v1.1.0';
+const DYNAMIC_CACHE = 'ukds-dynamic-v1.1.0';
+const IMAGE_CACHE = 'ukds-images-v1.1.0';
 
 // Files to cache immediately (critical resources)
 const STATIC_ASSETS = [