Extract translations from shared templates under layout domain (#2162)

[mitapujari-05]

Summary

Fixes #2162.

Updates Extract.php and Translation.php to extract translations from shared templates in _templates/ (header.php, alert.php, footer.php, legacy.php) and combine them under the single layout domain, rather than failing or creating separate template-specific translation files.

Changes

• _backend/Console/Extract.php:
  • Added translateLayout() to extract shared templates in a unified layout environment under the layout domain.
  • Added support for --page layout and mapped individual template inputs (e.g. --page _templates/header) to layout.
  • Configured user agent during extraction so legacy browser warning strings in legacy.php are included.
  • Output JSON with JSON_UNESCAPED_SLASHES to match standard formatting.
• _backend/Console/Translation.php:
  • Mapped all _templates/ paths from L10n::pages() into the single layout domain and deduplicated them.
  • Handled CLI --page argument so --page layout or --page _templates/... target the layout domain.
  • Normalized keys when preserving existing translations to ensure formatting whitespace differences do not drop existing localized strings.
  • Enforced LF (\n) line endings matching .editorconfig.

Testing

• Verified Extract.php --page layout extracts all strings from header.php, alert.php, footer.php, and legacy.php.
• Verified Translation.php -p layout updates translations under layout.json while preserving existing translations.
• Verified isolation of regular pages (e.g., index.php).
• Verified PSR-2 compliance with phpcs.
• Ran php _tests/translations.php (all 584 translation files passed, 0 errors).