feat(i18n): add Japanese (ja) support

[nyagasan]

Summary

This PR adds Japanese (ja) localization support to Beanconqueror.

Changes

• Added Japanese translations for the app UI
• Localized coffee-related terminology for brewing, cupping, beans, water, settings, statistics, and device integrations
• Translated AI import and language-detection strings for Japanese users
• Improved Japanese wording to make the localization natural and consistent
• Kept brand names, product names, and provider names in their original form where appropriate

Validation

• Verified the Japanese translation file is valid JSON
• Verified Japanese locale keys and structure match the English locale
• Confirmed remaining English strings are intentional proper nouns or product/provider names

resolve #1124

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}