[dryout]

What

Adds Japanese (ja) support, following docs/translating.md:

• Translate both catalogs (53 + 684 messages), keeping the source placeholders.
• Register 日本語 with Windows language detection and Yu Gothic UI.
• Add Japanese installer messages and the Settings window title used during upgrades. Set the Japanese installer font explicitly and keep the sensor explanation within the page.
• Add Japanese to the README, translation guide and manual testing checklist.

Testing

• Formatting, Clippy, catalog tests, debug and release builds, and installer packaging pass on Windows 11.
• Manually checked Japanese settings, details, language switching, settings search and installer pages.
• Tested fresh installation, silent upgrade, graceful closure of Japanese Settings, automatic restart and uninstall preserving settings in Windows Sandbox.
• The full workspace test run has one failure: object_without_instances_is_reported_as_unavailable. It also fails on the unmodified base commit on this machine.
• Installation of the optional sensor service and PawnIO was not tested.