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/components/admin/AdminClientTicketTab.vue

@@ -274,25 +274,22 @@ const availableStatusTransitions = computed(() => {
 })
 
 function getProjectName(iri: string): string {
-    const match = iri.match(/\/api\/projects\/(\d+)/)
-    if (!match) return ''
-    const id = Number(match[1])
+    const id = extractIdFromIri(iri)
+    if (!id) return ''
     return projects.value.find(p => p.id === id)?.name ?? ''
 }
 
 function getSubmitterName(iri: string | null): string {
     if (!iri) return '-'
-    const match = iri.match(/\/api\/users\/(\d+)/)
-    if (!match) return ''
-    const id = Number(match[1])
+    const id = extractIdFromIri(iri)
+    if (!id) return ''
     return users.value.find(u => u.id === id)?.username ?? ''
 }
 
 function getSubmitterUser(iri: string | null): UserData | undefined {
     if (!iri) return undefined
-    const match = iri.match(/\/api\/users\/(\d+)/)
-    if (!match) return undefined
-    const id = Number(match[1])
+    const id = extractIdFromIri(iri)
+    if (!id) return undefined
     return users.value.find(u => u.id === id)
 }