Add user columns, config, pagination, and filtering to admin overview

- New Username column (before Display name) and Email/Groups columns
- New conf/ (plugin had none): show_mail, show_grps, entrie

Add user columns, config, pagination, and filtering to admin overview

- New Username column (before Display name) and Email/Groups columns
- New conf/ (plugin had none): show_mail, show_grps, entries_per_page
- Numbered windowed pagination; per-column substring filters on all columns except Changed at
- Existing Setting drop-down kept, combined with text filters in one GET form (no JS)
- de/ru/ja settings translations, FF78-safe CSS, README and plugin.info.txt date updated

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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Review: fix JSON encoding, error message, locale cache, add translations

- helper.php: add JSON_UNESCAPED_UNICODE to json_encode flags
- lang/en: add %s placeholder to adminsavefail for consistency

Review: fix JSON encoding, error message, locale cache, add translations

- helper.php: add JSON_UNESCAPED_UNICODE to json_encode flags
- lang/en: add %s placeholder to adminsavefail for consistency with adminsaved
- action.php: reset plugin locale cache after applying user language override,
so the settings page renders in the user's chosen language rather than the
site default
- Add de, ru, ja translations
- plugin.info.txt: update date

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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