✨ 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/i18n/en.json

@@ -103,7 +103,10 @@
     "skill_tree_placeholder": "Interactive skill tree (coming soon)",
     "level": "Level",
     "project": "project",
-    "projects": "projects"
+    "projects": "projects",
+    "related_projects": "Projects using this skill",
+    "load_projects_error": "Unable to load related projects",
+    "no_related_projects": "No projects use this skill yet"
   },
   "pages": {
     "projects": {