Replace Flowbite stats YearPicker with in-house calendar

[KucharczykL]

Summary

• Replace the stats YearPicker Flowbite integration with an in-house four-column decade calendar.
• Reuse the shared drop-down/date-calendar positioning, dismissal, ARIA, and generated control-class machinery.
• Preserve the public YearPicker API, twelve-cell layout, decade bounds, available_years filtering, immediate navigation, and empty-URL guard.
• Remove games/static/js/datepicker.umd.js and stale runtime/source references.

Verification

• Full make check passed: 2,723 Python/browser tests.
• TypeScript suite passed: 754 tests.
• TypeScript compilation, mypy, Ruff, generated contracts, icons, and CSS checks passed.
• Focused YearPicker, date-range, and single-date browser tests passed.

[github-actions]

Staging deployment: https://timetracker-staging-feat-in-house-stats-year-picke.fly.dev