Export: Markdown export loses table structure, page links, databases and some formatting #238

Today

Markdown export works (src/lib/markdown/content-to-markdown.ts, added by PR #23, so #52 can be closed). These parts still lose information:

• Tables (lines 94–113):
  • The first row is always written as the header, even when the table has none.
  • Each cell is flattened to one line of text, so lists and to-dos in cells (TB1) are lost.
  • Merged cells shift the columns.
  • Column alignment is lost.
• Page links become 📄 Title with no link (line 187). They could link to the page's URL.
• Databases are dropped completely (line 207). An inline database could export as a Markdown table of its visible rows and columns (or a link to CSV export, Export a database view as CSV #56).
• Formatting: underline, highlight, text colour and sub/superscript are dropped (only bold, italic, strike, code and links are handled, lines 24–39). Highlight could be ==text==, underline <u>.
• Code blocks lose their filename (CB2) and line-number setting.
• Table of contents and breadcrumb are dropped. TOC could be a list of links to the headings.

Proposal

Fix each point above. For tables that can't be expressed in Markdown (merged cells, block content in cells), write an HTML <table>, which GitHub and most viewers render.

Done when

• A page with a header-less table, a table with lists in cells, a page link, an inline database and highlighted text exports to Markdown that renders correctly on GitHub.