fix: improve CSDN sync formatting

[Jennifer-ra]

Background

When syncing articles from CSDN browser pages to Zhihu, Juejin and WeChat Official Account, several formatting issues appeared even though the original CSDN page rendered correctly:

• Code block languages were lost or normalized incorrectly.
• Juejin code blocks without a detected language became bash.
• WeChat imported CSDN TOC/list structures with wrong order, empty list items, lost indentation, extra line breaks, inconsistent font sizes, and collapsed code line breaks.
• Juejin draft cover and summary were not populated.

Root Cause

CSDN-rendered HTML is display-friendly but not always editor-friendly. Some TOC/list structures contain nested or adjacent list markup that rich-text editors reinterpret differently. WeChat editor also sanitizes list styles and treats simple p/div/br blocks inside list items as separate paragraphs, which can create extra line breaks or flatten indentation during import.

Changes

• Preserve code block language from pre/code attributes and class names.
• Stop defaulting unknown fenced code language to bash.
• Add CSDN-specific extraction selectors and cleanup rules.
• Add list normalization for platform preprocessing.
• For WeChat:
  • Normalize list structures.
  • Convert CSDN TOC nested lists into stable single-level list items.
  • Preserve TOC visual indentation with text-level full-width spacing instead of editor-sanitized CSS indentation.
  • Flatten simple p/div/section/br wrappers inside list items.
  • Add stable font sizing for list and table content.
  • Normalize code blocks into line-preserving rich-text-friendly markup.
  • Use extension runtime fetch when downloading remote images before upload.
• For Zhihu:
  • Preserve code block language attributes.
  • Add list normalization.
  • Preserve blockquote semantics for Draft.js.
• For Juejin:
  • Upload and set draft cover image when available.
  • Fill brief_content from extracted article summary.
• Add tests for code language preservation and unknown-language handling.

Verification

• pnpm --filter @wechatsync/core typecheck
• pnpm --filter @wechatsync/extension typecheck
• pnpm --filter @wechatsync/core test -- --run
• pnpm --filter @wechatsync/extension build