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/utils/iri.ts

@@ -0,0 +1,11 @@
+/**
+ * Extract the numeric ID from an API Platform IRI string.
+ * Example: "/api/projects/5" → 5
+ */
+export function extractIdFromIri(iri: string | null | undefined): number {
+    if (!iri) return 0
+    const lastSlash = iri.lastIndexOf('/')
+    if (lastSlash === -1) return 0
+    const id = Number(iri.substring(lastSlash + 1))
+    return Number.isFinite(id) ? id : 0
+}