Add localization support for the LeafWiki UI

[perber]

Add localization/i18n support for the LeafWiki user interface so that UI text can be translated into languages other than English.

Problem

For team usage, English-only UI text can be a barrier, especially when not all colleagues are comfortable with English.

This affects adoption even if the wiki content itself is already written in the team's preferred language.

Scope

This issue is about localizing the LeafWiki application UI, for example:

• buttons
• labels
• navigation items
• dialogs
• search UI
• settings/admin UI
• system messages

This is not about translating wiki page content automatically.

Goal

Make LeafWiki UI strings translatable in a structured way, so that additional languages can be added incrementally.

A good first step would be to externalize UI strings and support language files for translations.

Possible approach

• introduce a basic i18n layer for frontend UI strings
• keep English as the default/fallback language
• load translations from locale files
• allow additional languages to be added later
• make it possible for community contributors to provide translations

Acceptance criteria

• UI text is no longer hardcoded throughout the app
• English remains the default/fallback language
• At least one additional language can be added through locale files
• The localization approach is documented
• It is possible to extend translations incrementally over time