feat(i18n): localize default journal title and extract from hardcoded values

[megatocha]

Preview

Context

The default journal title "My Field Guide" was hardcoded in the Java code. This meant players with non-English clients still saw the English title. This PR moves the default text into the translation system so it respects the player's language.

Summary of Changes

• Replaced hardcoded "My Field Guide" with a translatable lang key fieldguide.journal.default.title.
• Changed the internal default from the hardcoded string to null. When no custom title is set, the game now pulls the text from the client's current language file.
• Empty or blank titles are treated as null, so they correctly fall back to the translated default instead of saving an invisible custom name.
• Added the translation key to all existing lang files.
• Old save files that still contain the literal text "My Field Guide" are automatically converted to null on load, so they pick up the new translated default immediately.

Impact

• Saves: Fully backwards compatible. Existing worlds auto-migrate the old hardcoded title on next load.
• UI/Localization: The journal now shows the default title in the player's selected language.
• Multiplayer: No breaking changes. Empty strings are sent over the network when the title is unset, which the client resolves to the local translation.