fix(creator-studio): 沉浸式布局与公众号式写作体验

首轮实机体验暴露的问题：

- 投稿页仍在渲染主题头部的 all_top_fluid 挂件区、页脚与浮动入口，
  与工作台顶栏重复。header.php / footer.php 新增 jio_render_header、
  jio_render_footer、jio_render_fluid_widgets 过滤器，工作台模式下全部关闭
- 区块 Inspector 被写在 BlockList 之后，作为正文流的一部分渲染出
  一个巨大的「未选择区块」白框；移除内联 Inspector（区块设置改由
  行内工具栏的区块菜单提供）
- 中文正文被 wordcount 的 'words' 口径算成「字数：1」；改用
  characters_excluding_spaces，与前台 TinyMCE 字数一致
- 行宽 820px、标题 34px 偏松散；收窄到 720px / 27px，正文 16px/1.85，
  段落间距与选区描边收敛，行内工具栏改为深色圆角悬浮条

新增本地视觉验收脚本（不参与运行时）：
- scripts/preview.cjs：用模拟 bootstrap 生成离线预览页
- scripts/shoot.cjs：桌面/移动端截图与控制台错误采集

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

assets/creator-studio/build/index.css

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