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/nuxt.config.ts

@@ -23,14 +23,6 @@ export default defineNuxtConfig({
     devServer: {
         port: 3002,
     },
-    nitro: {
-        devProxy: {
-            '/api': {
-                target: 'http://nginx',
-                changeOrigin: true,
-            },
-        },
-    },
     components: [
         {path: '~/components', pathPrefix: false},
     ],