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/composables/useAvatarService.ts

@@ -5,11 +5,13 @@ export function useAvatarService() {
         const formData = new FormData()
         formData.append('file', file, 'avatar.png')
 
-        return $fetch(`/api/users/${userId}/avatar`, {
-            method: 'POST',
-            body: formData,
-            credentials: 'include',
-        })
+        return api.post<{ avatarUrl: string }>(
+            `/users/${userId}/avatar`,
+            formData as unknown as Record<string, unknown>,
+            {
+                toastSuccessKey: 'profile.avatarUpdated',
+            }
+        )
     }
 
     async function remove(userId: number): Promise<void> {