✨ Add skill projects modal with Headless UI (Story 2.5)

- Add GET /skills/{slug}/projects endpoint with level progression
- Install @headlessui/vue for accessible modal
- Create SkillProjectsModal with Dialog component:
  - Focus trap and keyboard navigation (automatic)
  - Fade + scale transitions with backdrop blur
  - prefers-reduced-motion support
- Create ProjectListItem with thumbnail and level display
- Integrate modal in competences.vue page
- Add translations for related projects UI

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

frontend/app/pages/competences.vue

@@ -66,6 +66,13 @@
         </p>
       </div>
     </div>
+
+    <!-- Skill projects modal -->
+    <FeatureSkillProjectsModal
+      :is-open="isModalOpen"
+      :skill="selectedSkill"
+      @close="closeModal"
+    />
   </div>
 </template>
 
@@ -99,10 +106,21 @@ function getCategoryIcon(category: string): string {
   return categoryIcons[category.toLowerCase()] ?? '📚'
 }
 
-// Handle skill click (preparation for Story 2.5)
+// Modal state
+const isModalOpen = ref(false)
+const selectedSkill = ref<Skill | null>(null)
+
 function handleSkillClick(skill: Skill) {
-  // Will be implemented in Story 2.5 - modal with related projects
-  console.log('Skill clicked:', skill.slug)
+  selectedSkill.value = skill
+  isModalOpen.value = true
+}
+
+function closeModal() {
+  isModalOpen.value = false
+  // Keep selectedSkill for close animation
+  setTimeout(() => {
+    selectedSkill.value = null
+  }, 300)
 }
 
 onMounted(() => {