feat(export): add missing chapter placeholder rendering

[saudadez21]

Description

This PR introduces a new configuration option render_missing_chapter and adds unified “missing chapter placeholder” support across TXT, EPUB, and HTML exporters.

When a chapter is missing (e.g., inaccessible, unsubscribed, or not downloaded), and render_missing_chapter = true, exporters now insert a placeholder section instead of silently skipping the chapter.

This improves readability, prevents unnoticed gaps, and provides a consistent experience across formats.

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Changes

• Added new config flag:
  render_missing_chapter: bool = true controls whether missing chapters should render a placeholder during export.
• Implemented _xp_epub_missing_chapter() helper and integrated the logic into:
  • _export_volume_epub
  • _export_book_epub
• Implemented _xp_html_missing_chapter() helper and integrated into:
  • _export_book_html
• Implemented _xp_txt_missing_chapter() helper and integrated into:
  • _export_book_txt
• Ensured all formats use a consistent placeholder structure:
  • Title line
  • "本章内容暂不可用" fallback text
• Updated export pipelines to correctly handle missing chapters instead of skipping them silently.

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Motivation

Previously, missing or inaccessible chapters were silently omitted during export.

For long novels or novels with many paid chapters, this often resulted in:

• unnoticed chapter gaps
• inconsistent reading experience
• difficulty identifying missing content

By adding explicit placeholder rendering, users can immediately see where chapters are unavailable, making the exported files more complete and easier to navigate.

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Related Issues

Fixes #148

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Type of change

• New feature (non-breaking change which adds functionality)