feat(creator-studio): 公众号式插入面板与封面条

- 新增常驻「添加内容」按钮与插入面板：分组网格列出正文/标题/列表/引用、
  图片/视频/嵌入、代码/分隔线/表格/付费内容；条目不满足能力时不出现在面板里
- 面板支持方向键在网格内移动焦点、Esc 关闭；桌面居中浮层，移动端贴底抽屉并
  留出安全区
- 空文档插入区块时替换初始空白段落，不留空行
- 新增封面条：无封面时给低调入口，有封面时显示 2.35:1 缩略图，悬停出现
  「更换封面」，右上角可移除
- 补充契约测试：头部/页脚/挂件区开关、插入面板真实区块名与能力过滤、
  中文字数口径

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/creator-studio/src/index.tsx

@@ -9,9 +9,9 @@
  */
 
 import { createRoot, useCallback, useEffect, useMemo, useRef, useState } from '@wordpress/element';
-import { parse, serialize } from '@wordpress/blocks';
+import { createBlock, parse, serialize } from '@wordpress/blocks';
 import { registerCoreBlocks } from '@wordpress/block-library';
-import { useSelect } from '@wordpress/data';
+import { useDispatch, useSelect } from '@wordpress/data';
 import { BlockEditorProvider, BlockList, BlockTools } from '@wordpress/block-editor';
 import { count as countWords } from '@wordpress/wordcount';
 import { __ } from '@wordpress/i18n';
@@ -20,6 +20,9 @@ import { apiPost } from './services/api';
 import { installApiFetchNonce } from './services/api-fetch';
 import { installMediaUploadBridge } from './services/media';
 import { PublishDrawer } from './panels/publish-drawer';
+import { CoverStrip } from './components/cover-strip';
+import { InsertPanel, type InsertItem } from './panels/insert-panel';
+import { buildInsertGroups } from './utils/insert-items';
 import { SaveState, type SaveStateValue } from './components/save-state';
 import { useAutosave } from './hooks/use-autosave';
 import { useLocalRecovery } from './hooks/use-local-recovery';
@@ -106,6 +109,7 @@ function Studio(): JSX.Element {
 	const [blocks, setBlocks] = useState(() => parse(draft.content || ''));
 	const [saveState, setSaveState] = useState<SaveStateValue>({ state: 'ready', at: '' });
 	const [drawerOpen, setDrawerOpen] = useState(false);
+	const [insertOpen, setInsertOpen] = useState(false);
 	const [notice, setNotice] = useState<{ tone: 'ok' | 'error'; text: string } | null>(null);
 	const [recovery, setRecovery] = useState<null | { savedAt: string }>(null);
 	const dirtyRef = useRef(false);
@@ -328,9 +332,26 @@ function Studio(): JSX.Element {
 		[]
 	);
 
+	const insertGroups = useMemo(() => buildInsertGroups(capabilities), [capabilities]);
+
+	const insertBlock = useCallback(
+		(item: InsertItem) => {
+			setInsertOpen(false);
+			dirtyRef.current = true;
+			setBlocks((current: any[]) => {
+				// 空文档时直接替换初始的空白段落，避免留下一个空行。
+				const next = [...current];
+				if (next.length === 1 && next[0].name === 'core/paragraph' && !next[0].attributes?.content) {
+					return [createBlock(item.name, item.attributes || {})];
+				}
+				return [...next, createBlock(item.name, item.attributes || {})];
+			});
+		},
+		[]
+	);
+
 	return (
-		<div className="jio-creator" data-status={bootstrap.status || ''} data-blocks={selectedBlockCount}>
-			<header className="jio-creator__bar">
+		<div className="jio-creator" data-status={bootstrap.status || ''} data-blocks={selectedBlockCount}>			<header className="jio-creator__bar">
 				<a className="jio-creator__back" href={bootstrap.urls?.center || '/'}>
 					<i className="fa fas fa-angle-left" aria-hidden="true" />
 					<span>{__('返回', 'jio')}</span>
@@ -371,6 +392,13 @@ function Studio(): JSX.Element {
 			)}
 
 			<main className="jio-creator__canvas">
+				<CoverStrip
+					featured={draft.featured}
+					canEdit={!!capabilities.canUploadImage}
+					onOpen={() => setDrawerOpen(true)}
+					onClear={() => patchDraft({ featured: { type: '', data: {} } })}
+				/>
+
 				<label className="jio-creator__title">
 					<span className="screen-reader-text">{__('文章标题', 'jio')}</span>
 					<textarea
@@ -407,6 +435,18 @@ function Studio(): JSX.Element {
 							<BlockList />
 						</BlockTools>
 					</BlockEditorProvider>
+
+					{/* 公众号式常驻插入按钮：始终停在正文末尾，点击打开内容面板。 */}
+					<button
+						type="button"
+						className="jio-creator__insert"
+						onClick={() => setInsertOpen(true)}
+						aria-haspopup="dialog"
+						aria-expanded={insertOpen}
+					>
+						<i className="fa fas fa-plus" aria-hidden="true" />
+						<span>{__('添加内容', 'jio')}</span>
+					</button>
 				</div>
 
 				<footer className="jio-creator__footer">
@@ -417,6 +457,14 @@ function Studio(): JSX.Element {
 				</footer>
 			</main>
 
+			{insertOpen && (
+				<InsertPanel
+					groups={insertGroups}
+					onInsert={insertBlock}
+					onClose={() => setInsertOpen(false)}
+				/>
+			)}
+
 			{drawerOpen && (
 				<PublishDrawer
 					draft={draft}