feat(ui): use modal dialogs for entity creation and editing

Introduce reusable Modal component for forms
Update router to preserve scroll position when toggling modals
Refactor admin and entity views to render editors as overlays

frontend/src/i18n.ts

@@ -13,6 +13,7 @@ const messages = {
       back: 'Back',
       backToList: 'Back to list',
       cancel: 'Cancel',
+      close: 'Close',
       create: 'Create',
       delete: 'Delete',
       edit: 'Edit',
@@ -266,6 +267,7 @@ const messages = {
       back: '返回',
       backToList: '返回列表',
       cancel: '取消',
+      close: '关闭',
       create: '创建',
       delete: '删除',
       edit: '编辑',