feat: add Helpers::formatAnnouncement() for announcement-bar ACF group

[parisek]

Summary

• Add Helpers::formatAnnouncement(?array $value): array to handle the announcement-bar ACF group, upstreaming logic found byte-identical as a private get_announcement() in four downstream projects (sloneek, mairateam, proficio, proficiohub) plus starter_theme
• Takes the raw ACF group value (enabled, text, dates.date_from/date_to as date_picker 'U' timestamps = midnight UTC), re-anchors the timestamps to wp_timezone() day bounds (00:00:00 for date_from, 23:59:59 for date_to), and returns millisecond timestamps for Alpine.js
• Disabled/absent input yields the empty shape; downstream reduces to Helpers::formatAnnouncement($global_fields['announcement'] ?? null)
• Add 5 new tests covering DST-timezone re-anchoring and ACF string-timestamp inputs (845 total green); PHPStan level 8 clean

Changes

• src/Helpers.php (modified)
• tests/Unit/Helpers/FormatAnnouncementTest.php (added)
• CHANGELOG.md (modified)
• README.md (modified)

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