feat(mobile): add reading settings, manual theme and immersive header

- add a Reading settings screen (content font, line height, text color,
  inline style, code theme) reachable from Appearance and from the entry
  "more" menu
- add a System / Light / Dark theme setting applied through
  Appearance.setColorScheme so the reader WebView follows it as well
- optionally slide the entry header away while scrolling down and keep a
  blurred status bar strip so the clock stays readable
- forward reader typography to the html renderer through a new
  setReaderStyle bridge call

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>