feat(05-11): add adminDeleteUser API function + inline delete confirmation panel

- Export adminDeleteUser(id, adminPassword) from client.js — sends JSON body to DELETE /api/admin/users/{id}
- AdminUsersTab: add confirmDelete, deletePassword, deleteError state refs
- AdminUsersTab: add startDelete, cancelDelete, confirmDoDelete functions (mutually exclusive with deactivate panel)
- AdminUsersTab: Delete button added to active and deactivated user rows
- AdminUsersTab: inline password confirmation panel with Argon2 verification via backend

frontend/src/api/client.js

@@ -269,6 +269,14 @@ export function adminUpdateAiConfig(id, provider, model) {
   })
 }
 
+export function adminDeleteUser(id, adminPassword) {
+  return request(`/api/admin/users/${id}`, {
+    method: 'DELETE',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ admin_password: adminPassword }),
+  })
+}
+
 // ── Folders ───────────────────────────────────────────────────────────────────
 
 export function listFolders(parentId = null) {