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) {

frontend/src/components/admin/AdminUsersTab.vue

@@ -171,6 +171,43 @@
                 </div>
               </div>
 
+              <!-- Inline delete confirmation panel -->
+              <div v-else-if="confirmDelete === user.id" class="space-y-2">
+                <p class="text-xs text-red-700 font-semibold">
+                  Permanently delete <span class="font-bold">{{ user.email }}</span>?
+                  This will erase all their documents, cloud connections, and quota data. This cannot be undone.
+                </p>
+                <div>
+                  <label class="block text-xs text-gray-700 mb-1 font-semibold">Your admin password to confirm</label>
+                  <input
+                    v-model="deletePassword"
+                    type="password"
+                    autocomplete="current-password"
+                    placeholder="Admin password"
+                    class="block w-full rounded-lg px-2 py-1.5 text-xs border border-red-300 focus:outline-none focus:ring-2 focus:ring-red-500 focus:border-red-500 transition-colors"
+                    @keydown.enter.prevent="confirmDoDelete(user.id)"
+                  />
+                </div>
+                <p v-if="deleteError" class="text-xs text-red-600">{{ deleteError }}</p>
+                <div class="flex items-center gap-2">
+                  <button
+                    @click="confirmDoDelete(user.id)"
+                    :disabled="pendingAction[user.id] || !deletePassword"
+                    class="text-red-700 hover:text-red-800 text-sm font-semibold disabled:opacity-50"
+                  >
+                    <span v-if="pendingAction[user.id]" class="flex items-center gap-1">
+                      <span class="animate-spin rounded-full border-2 border-current border-t-transparent w-3 h-3"></span>
+                      Deleting…
+                    </span>
+                    <span v-else>Delete permanently</span>
+                  </button>
+                  <span class="text-gray-300">·</span>
+                  <button @click="cancelDelete" class="text-gray-500 hover:text-gray-700 text-sm">
+                    Cancel
+                  </button>
+                </div>
+              </div>
+
               <!-- Normal actions -->
               <div v-else class="flex items-center gap-2">
                 <span v-if="pendingAction[user.id]" class="flex items-center gap-1 text-gray-400 text-sm">
@@ -191,6 +228,13 @@
                   >
                     Deactivate
                   </button>
+                  <span class="text-gray-300">·</span>
+                  <button
+                    @click="startDelete(user.id)"
+                    class="text-red-800 hover:text-red-900 text-sm font-semibold"
+                  >
+                    Delete
+                  </button>
                 </template>
 
                 <template v-else>
@@ -200,6 +244,13 @@
                   >
                     Reactivate
                   </button>
+                  <span class="text-gray-300">·</span>
+                  <button
+                    @click="startDelete(user.id)"
+                    class="text-red-800 hover:text-red-900 text-sm font-semibold"
+                  >
+                    Delete
+                  </button>
                 </template>
               </div>
             </td>
@@ -221,6 +272,9 @@ const users = ref([])
 const loading = ref(false)
 const showCreateForm = ref(false)
 const confirmDeactivate = ref(null)
+const confirmDelete = ref(null)
+const deletePassword = ref('')
+const deleteError = ref(null)
 const pendingAction = reactive({})
 const actionError = ref(null)
 const creating = ref(false)
@@ -308,6 +362,36 @@ async function submitCreate() {
 
 function startDeactivate(id) {
   confirmDeactivate.value = id
+  confirmDelete.value = null
+  deletePassword.value = ''
+  deleteError.value = null
+}
+
+function startDelete(id) {
+  confirmDelete.value = id
+  deletePassword.value = ''
+  deleteError.value = null
+  confirmDeactivate.value = null
+}
+
+function cancelDelete() {
+  confirmDelete.value = null
+  deletePassword.value = ''
+  deleteError.value = null
+}
+
+async function confirmDoDelete(id) {
+  pendingAction[id] = true
+  deleteError.value = null
+  try {
+    await api.adminDeleteUser(id, deletePassword.value)
+    users.value = users.value.filter(u => u.id !== id)
+    cancelDelete()
+  } catch (e) {
+    deleteError.value = e.message
+  } finally {
+    delete pendingAction[id]
+  }
 }
 
 async function confirmDoDeactivate(id) {