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>