feat(17-02): tree reorder page and singleton form in the admin

- ReorderView: indented tree with drag handle, move up/down, outdent and
  indent (buttons and arrow keys on the handle), pointer drag before, after
  or inside a row and never into its own subtree; one POST with the whole
  node set 400 ms after the last move, flushed on leave; a failed save
  restores the saved tree, a 409 reloads; loading, empty, load-failure and
  403 states; partition selector
- ListView: outline Reorder button before New record, passing the filtered
  partition
- SettingsIndexView opens a singleton controller's record through
  GET …/singleton; FormView in singleton mode has no Delete and returns to
  the settings index
- reorder.* and messages.list.reorder phrase keys (en, pl), smoke spec, docs

admin/src/app/router.ts

@@ -8,6 +8,7 @@ import LoginView from '../views/LoginView.vue'
 import ListView from '../views/ListView.vue'
 import FormView from '../views/FormView.vue'
 import PreviewView from '../views/PreviewView.vue'
+import ReorderView from '../views/ReorderView.vue'
 import SettingsFormView from '../views/SettingsFormView.vue'
 import SettingsIndexView from '../views/SettingsIndexView.vue'
 import AccountView from '../views/AccountView.vue'
@@ -36,7 +37,7 @@ export function safeRedirect(value: unknown): string | null {
   return value
 }
 
-const CONTROLLER_ROUTES = new Set(['list', 'create', 'record', 'preview'])
+const CONTROLLER_ROUTES = new Set(['list', 'create', 'record', 'preview', 'reorder'])
 
 /**
  * The controller a route shows, or '' for every other screen (settings,
@@ -72,6 +73,8 @@ export function createAdminRouter(history: RouterHistory = createWebHistory(runt
       { path: '/account', name: 'account', component: AccountView, meta: { shell: true } },
       { path: '/:vendor/:plugin/:controller', name: 'list', component: ListView, meta: { shell: true } },
       { path: '/:vendor/:plugin/:controller/create', name: 'create', component: FormView, meta: { shell: true } },
+      // The tree reorder page of a list with config_reorder.yaml (D-09).
+      { path: '/:vendor/:plugin/:controller/reorder', name: 'reorder', component: ReorderView, meta: { shell: true } },
       { path: '/:vendor/:plugin/:controller/:id(\\d+)', name: 'record', component: FormView, meta: { shell: true } },
       // The read-only record screen of a form with a preview block (D-11).
       {