Tighten the Community Activity view

[zixiaowang17]

Summary

• remove the implementation-detail sentence above Community Activity
• trim leading weeks with no activity, retaining one empty week as visual context
• keep the overview and every contributor chart on the same trimmed time axis
• replace the misleading fixed 52-week label with the contributor count
• tighten the spacing below the Community Activity heading
• bump static asset versions so the corrected chart is visible immediately after deployment

With the current snapshot, the visible chart changes from 52 weeks to 20 weeks.

Verification

• python3 tools/build_homepage_data.py --validate-only
• python3 -m unittest discover -s tools -p "test_*.py"
• python3 tools/build_site.py
• node --check homepage.js
• git diff --check