v2.1.1 — viewer rendering: tables, dark mode, sanitizer gaps

Viewer rendering round — table/dark-mode/element styling from a user report on mobile.

Fixed

• GFM strikethrough (~~text~~) survives sanitization — <del> is not in
  sanitize-html's default allowlist and was silently dropped, losing the strikethrough
  meaning entirely
• Table column alignment survives sanitization — marked emits GFM :--:/--: as a
  presentational align attribute on th/td, which the sanitizer stripped
• Tables render with real borders (border-collapse, cell padding, header background)
  instead of unstyled runs of text
• Wide tables scroll horizontally (display:block; overflow-x:auto) instead of bursting
  the layout on phones
• Dark mode: color-scheme: light dark plus a prefers-color-scheme: dark token set —
  the page, and the password form's input/button, now follow the system theme instead of
  always rendering white
• blockquote (left border + muted text) and img (max-width:100%; height:auto) are
  styled; large images no longer overflow on mobile

Changed

• Minor polish: inline code gets padding + radius (reset inside pre), kbd renders
  as a bordered keycap, task-list checkboxes drop the redundant list bullet
• Document <style>/style attributes remain stripped by design — styling belongs to
  the page template, content carries semantics only