feat: guider box tracking + pick overlay X per target mode, white annotations

- guider.py: update box_center after each fit so the search box tracks
  the star across corrections
- guider-panel.js: draw X on overlay for center/stick/custom after
  right-click (non-destructive) and on Target dropdown change;
  fix left-click phase-2 to set TX/TY before calling onTargetChange;
  overlay annotations (box, X, dashed line) now white
- coordinates.py: reformat multiline log.debug fstring (cosmetic)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>