Saguinus v0.2.3

Saguinus v0.2.3

• Strikethrough (~~text~~) — type, paste, and toggle via the toolbar.
• Math support (KaTeX) — inline ($...$) and block ($$...$$) math,
  with editable LaTeX source and dedicated toolbar buttons.
• Task lists — - [ ] / - [x] checkboxes that render as clickable
  boxes and round-trip through markdown.
• Markdown help panel — a new "Markdown Guide" in the activity bar with
  a quick reference for every supported markdown construct, including the
  image resize/alignment syntax.
• Fixes — markdown paste/copy now round-trips properly, heading/paragraph
  spacing restored, tab switching no longer lags, page-count and editor page
  width now match the preview/PDF.

See v0.2.3.md for the full list of changes.