feat: add language selector and indicator to page creation

[Pavel910]

Summary

• Add language selector to the "Create Page" dialog, driven by the languages module. Shows a dropdown when multiple languages exist, auto-assigns when only one is configured, hidden otherwise.
• Display a language code tag (LanguageCodeTag) in the page editor title bar and pages list table.
• Refactor CreatePage dialog into a standalone CreatePageDialog registered via AdminConfig.Dialog, replacing the inline useDialogs-based approach.
• Extract reusable PageType.Language, PageType.Title, and PageType.Path sub-components for composing page type forms.
• Add shared useLanguages hook and LanguageSelector component; deduplicate language fetching in TranslatePageDialog.
• Fix concurrent language fetches in ListLanguagesRepository with request deduplication.

Test plan

• Create a page with multiple languages configured — verify language dropdown appears and selected language is prepended to the path
• Create a page with a single language — verify no dropdown, language is auto-assigned
• Create a page with no languages — verify no language UI appears
• Open page editor — verify language code tag shows next to the title
• Translate a page — verify the new page copy is in the selected folder and the right language assigned

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