Blackboard Text 2.4.1

New drawing tools, page history, and PNG/PDF export. Includes the interface
pass prepared as 2.3.3 and the feature work prepared as 2.4.0. Short links
need the 2.4 short-link service, which accepts the new format.

Added

• A small drawing toolbar beside page navigation: pen, marker, eraser and
  select, then Undo and More. Opening it never moves your text, changes
  wrapping or jumps the scroll position. On phones it sits above page
  navigation.
• Tap the active tool for its ink: six sizes, theme ink, four chalk colours,
  a custom colour and your recent colours, plus the tool's own switch
  (pressure for the pen, whole-stroke erasing, delete for a selection).
• Shift draws straight lines and Alt erases temporarily. Select moves or
  restyles a stroke. Drawing Redo. Each tool remembers its own settings.
• More → Attach new strokes to words pins an annotation to a word: typing
  before the word moves it along; deleting the word turns it into a free
  stroke instead of deleting it.
• Export PNG or PDF from Settings → Page actions, with text and
  drawings. Long pages become a multipage PDF. The PDF is an image of the
  board, so its text cannot be selected; use TXT or Markdown for that.
• Page history keeps up to 25 versions of each page (100 in total,
  10 MB) in this browser. Recovering a version creates a new page.
• Pin pages; search results show a snippet of the matching text.
• Describe drawings for people who read a shared copy with a screen reader.
• Link management backup (Settings → Your data) exports the keys that
  disable your short links, encrypted with a passphrase, so you can manage
  them from another browser. It contains no note text.
• Interface language: English or Russian. Your own text is never translated.

Changed

• New strokes use one smoother renderer in the editor, the reader and exports.
  Older strokes keep their look until More → Smooth existing, which Undo
  can reverse.
• Drawings stay sharp however long the page gets.
• Settings keeps its title and Reset in place while the middle scrolls, and
  runs Pages → Theme → Layout → Colours → Your data.
• Dialogs are centred and share one style; Delete and Clear confirmations are
  opaque. One stroked icon set, press feedback on every control, and short
  motion that respects Reduce Motion.
• New backups and shared notes use format version 2. This release still opens
  version-1 backups and links; older apps refuse version-2 files instead of
  silently dropping drawing details, so update every copy you use.

Fixed

• Reloading no longer jumps the note to an old scroll position, and a late
  scroll restore can't override a scroll you just made.
• The colour picker and font menu no longer close when the pointer leaves them.
• Save errors are tracked per operation; a successful save clears the warning.