✨ 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/package.json

@@ -10,6 +10,7 @@
     "postinstall": "nuxt prepare"
   },
   "dependencies": {
+    "@headlessui/vue": "^1.7.23",
     "@nuxt/image": "^1.9.0",
     "@nuxtjs/i18n": "^9.0.0",
     "@nuxtjs/sitemap": "^7.2.0",