Annotated screenshots in lightbox; region/whole-page capture in extension ...

- Extract shared shapeDraw.ts (pen/arrow/rect/text) used by both the
  annotation editor and the viewer.
- ScreenshotViewer lightbox now renders a canvas with the screenshot plus
  annotation shapes baked in, so opening a screenshot fullscreen shows the
  drawn version instead of the raw image.
- New RegionSelector step in the extension: after picking an element, the
  page is captured pristine (before any overlay renders) and the user can
  either drag a region on the page (cropped at devicePixelRatio) or take
  the whole page via the button / Enter.
- Whole-page capture scrolls and stitches viewport tiles, hiding the
  overlay and fixed/sticky elements; per-tile captures retry with backoff
  because captureVisibleTab is throttled.
- NoteComposer receives the screenshot from the region selector instead of
  capturing on its own; e2e updated (Enter = whole page).

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