✨ 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>

docs/implementation-artifacts/sprint-status.yaml

@@ -61,7 +61,7 @@ development_status:
   2-2-page-projets-galerie: review
   2-3-page-projet-detail: review
   2-4-page-competences-affichage-categories: review
-  2-5-competences-cliquables-projets-lies: ready-for-dev
+  2-5-competences-cliquables-projets-lies: review
   2-6-page-temoignages-migrations-bdd: ready-for-dev
   2-7-composant-dialogue-pnj: ready-for-dev
   2-8-page-parcours-timeline-narrative: ready-for-dev