Make the annotation editor comfortable to draw in ...

- AnnotationEditor gains a fit="contain" mode: the canvas scales to fit
  its box entirely instead of being cropped to container width.
- Report page: "Edit annotations" now opens a fullscreen editor overlay
  (toolbar + large canvas + Save/Cancel, Escape cancels) instead of the
  small inline editor.
- Extension composer: widened to 920px and the screenshot area now takes
  55vh with the canvas contain-fit, so there is actual room to draw.

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

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>