…d multi-locale routes

- Add reject competition flow with reason (admin panel, handler, email notification)
- Send email notifications on competition submit, approve, and reject
- Add return_url/return_title navigation to all event links on /events page
- Add return back button to add/edit competition and event detail pages
- Add es/ja/fr/de locale routes for all competition management controllers
- Filter rejected competitions from pending approvals list
- Add rejection state display on edit competition page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Add required validation for location and dates on offline events
- Add feature flags (admin-only) for table layout and stopwatch
- Add participant management date notice (expected 7.4.2026)
- Improve round form: single-day events show date + time-only picker
- Add timezone select with country-based defaults and UTC conversion
- Add Coloris color picker plugin replacing native color inputs
- Add country-to-timezone mapping on CountryCode enum

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The round puzzle form was missing the time-form-autocomplete Stimulus
controller, so brand→puzzle filtering and new-puzzle field toggling
did not work. Made competition target optional in the JS controller
since the round form doesn't have one.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>