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>

tests/widget-system/creator-studio-contract.test.cjs

@@ -142,3 +142,39 @@ test('creator studio build artifacts and contracts exist', () => {
 	assert.ok(exists('src/Core/PreviewToken.php'));
 	assert.ok(exists('src/Admin/Native/SubmissionReview.php'));
 });
+
+test('沉浸式布局屏蔽主题头部/页脚与挂件区', () => {
+	const header = read('header.php');
+	const footer = read('footer.php');
+	// 工作台模式下这些输出必须可关闭，否则顶栏重复、正文被挂件挤压。
+	assert.match(header, /apply_filters\('jio_render_header', true\)/);
+	assert.match(header, /apply_filters\('jio_render_fluid_widgets', true\)/);
+	assert.match(footer, /apply_filters\('jio_render_footer', true\)/);
+	assert.match(pageSubmit, /add_filter\('jio_render_header', '__return_false'\)/);
+	assert.match(pageSubmit, /add_filter\('jio_render_footer', '__return_false'\)/);
+	assert.match(pageSubmit, /add_filter\('jio_render_fluid_widgets', '__return_false'\)/);
+});
+
+test('插入面板与封面条走真实区块名与服务端能力', () => {
+	const insertItems = read('assets/creator-studio/src/utils/insert-items.ts');
+	const insertPanel = read('assets/creator-studio/src/panels/insert-panel.tsx');
+	const cover = read('assets/creator-studio/src/components/cover-strip.tsx');
+
+	assert.match(appSource, /createBlock/);
+	assert.match(insertItems, /'core\/paragraph'/);
+	assert.match(insertItems, /'core\/heading'/);
+	assert.match(insertItems, /'core\/image'/);
+	assert.match(insertItems, /'jio\/paid-fragment'/);
+	// 没有能力的条目不能出现在面板里。
+	assert.match(insertItems, /if \(capabilities\.canUploadImage\)/);
+	assert.match(insertItems, /if \(capabilities\.canPay\)/);
+	assert.match(insertPanel, /role="dialog"/);
+	assert.match(insertPanel, /event\.key === 'Escape'/);
+	assert.match(cover, /添加封面/);
+	assert.match(appSource, /<CoverStrip/);
+});
+
+test('中文字数按字符统计而非按空格分词', () => {
+	assert.match(appSource, /characters_excluding_spaces/);
+	assert.doesNotMatch(appSource, /countWords\([^)]*'words'/);
+});