✨ 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/composables/useFetchSkillProjects.ts

@@ -0,0 +1,39 @@
+import type { Skill } from '~/types/skill'
+
+export interface SkillProject {
+  id: number
+  slug: string
+  title: string
+  short_description: string
+  image: string
+  date_completed: string | null
+  level_before: number
+  level_after: number
+  level_description: string | null
+}
+
+interface SkillProjectsResponse {
+  data: {
+    skill: Pick<Skill, 'id' | 'slug' | 'name' | 'description' | 'level' | 'max_level'>
+    projects: SkillProject[]
+  }
+  meta: { lang: string }
+}
+
+export function useFetchSkillProjects(slug: Ref<string | null>) {
+  const config = useRuntimeConfig()
+  const { locale } = useI18n()
+
+  return useFetch<SkillProjectsResponse>(
+    () => slug.value ? `/skills/${slug.value}/projects` : '',
+    {
+      baseURL: config.public.apiUrl as string,
+      headers: {
+        'X-API-Key': config.public.apiKey as string,
+        'Accept-Language': locale.value,
+      },
+      immediate: false,
+      watch: false,
+    },
+  )
+}