feat: Build Experience section with timeline (Task 8)

- Create Experience.tsx component with vertical timeline layout
- Add 5 roles from NHS and Tesco with bullet points
- ECG waveform SVG decoration beside heading
- Timeline dots filled for current roles
- Hover effects on cards (scale, shadow, left border)
- Scroll-triggered animations using useScrollReveal hook
- Responsive: hide timeline line/dots on mobile
- Fix useScrollReveal ref type for React 18+ compatibility

src/hooks/useScrollReveal.ts

@@ -8,9 +8,9 @@ interface UseScrollRevealOptions {
 
 export function useScrollReveal<T extends HTMLElement>(
   options: UseScrollRevealOptions = {}
-): [RefObject<T | null>, boolean] {
+): [RefObject<T>, boolean] {
   const { threshold = 0.15, rootMargin = '0px', triggerOnce = true } = options
-  const ref = useRef<T | null>(null)
+  const ref = useRef<T>(null)
   const [isVisible, setIsVisible] = useState(false)
 
   useEffect(() => {