✨ 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-lock.json

@@ -7,6 +7,7 @@
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@@ -1053,6 +1054,20 @@
         "node": ">=14"
       }
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+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
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+      "funding": {
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+        "url": "https://github.com/sponsors/tannerlinsley"
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+      "peerDependencies": {
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