Expand measurable criteria for H6-H10 heuristics

[mbilgesu]

Hey @marcgc21 @KarinePistili — hope you're doing well!
While going through the codebase I noticed that H6--H10 in app/core/constants.py each had only 2 measurable criteria, while H1--H5 have 3--5 each. Since the README marks H4--H10 completion as a Phase 2 target, I went ahead and expanded the criteria to bring H6--H10 to a consistent level.
Changes:

• H6: added H6.3 (breadcrumb / location awareness) and H6.4 (recently used items)
• H7: added H7.3 (search and filter for power users) and H7.4 (bulk operations)
• H8: added H8.3 (visual hierarchy guides attention) and H8.4 (color and contrast)
• H9: added H9.3 (error location highlighted) and H9.4 (user data preserved on error)
• H10: added H10.3 (contextual tooltips) and H10.4 (onboarding for first-time users)
  All new criteria follow the existing structure (id, description, evaluation, severity_weights) and are consistent with Nielsen's original definitions. No breaking changes — existing criteria and severity weights are untouched.
  Let me know if you'd like any adjustments!