feat(frontend) : admin middleware, fix avatar upload, centralize IRI extraction, remove Nitro proxy

- Add admin middleware protecting /admin page (ROLE_ADMIN check)
- Fix useAvatarService to use useApi() with FormData detection
- Create extractIdFromIri() utility, replace manual IRI parsing
- Remove redundant Nitro devProxy (Vite proxy handles dev)

Tickets: T-014, T-015, T-017, T-021

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/middleware/admin.ts

@@ -0,0 +1,7 @@
+export default defineNuxtRouteMiddleware(() => {
+    const auth = useAuthStore()
+
+    if (!auth.isAuthenticated || !auth.user?.roles?.includes('ROLE_ADMIN')) {
+        return navigateTo('/')
+    }
+})