✨ Add journey timeline page with scroll animations (Story 2.8)

- Create useIntersectionObserver composable for scroll-triggered animations
- Add TimelineItem component with alternating layout (desktop)
- Implement journey page with i18n-based milestones data
- Add 7 career milestones (2018-2025) in FR and EN
- Gradient timeline line with animated connection points
- Glassmorphism card design with hover effects
- Respect prefers-reduced-motion for all animations

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

frontend/app/composables/useIntersectionObserver.ts

@@ -0,0 +1,57 @@
+export interface UseIntersectionObserverOptions {
+  threshold?: number
+  rootMargin?: string
+  once?: boolean
+}
+
+export function useIntersectionObserver(
+  target: Ref<HTMLElement | null>,
+  options: UseIntersectionObserverOptions = {}
+) {
+  const { threshold = 0.1, rootMargin = '0px', once = true } = options
+
+  const isVisible = ref(false)
+
+  if (import.meta.client) {
+    let observer: IntersectionObserver | null = null
+
+    const cleanup = () => {
+      if (observer) {
+        observer.disconnect()
+        observer = null
+      }
+    }
+
+    watch(
+      target,
+      (el) => {
+        cleanup()
+
+        if (!el) return
+
+        observer = new IntersectionObserver(
+          (entries) => {
+            entries.forEach((entry) => {
+              if (entry.isIntersecting) {
+                isVisible.value = true
+                if (once && observer) {
+                  observer.unobserve(entry.target)
+                }
+              } else if (!once) {
+                isVisible.value = false
+              }
+            })
+          },
+          { threshold, rootMargin }
+        )
+
+        observer.observe(el)
+      },
+      { immediate: true }
+    )
+
+    onUnmounted(cleanup)
+  }
+
+  return { isVisible: readonly(isVisible) }
+}