feat(i18n): add Japanese localization

[yuu1111]

Summary

• Add Japanese localization for the full application UI
• Detect Japanese browser locale automatically
• Add Japanese to the language selector
• Fill in missing entries found in the existing locales while adding Japanese support
• Allow localized settings navigation labels to wrap within the sidebar

Test plan

• pnpm typecheck
• pnpm test
• Verify locale key and placeholder parity
• Build and run the Tauri application locally
• Verify Japanese language selection and settings navigation layout

[itvincent-git]

Thanks for adding Japanese localization! The overall implementation looks good.

I tested the PR against the latest main: typecheck, tests, production build, and placeholder parity all pass.

Before merging, could you please update/rebase the branch onto the latest main and add the following recently introduced Japanese locale
entries?

• sessions.quota.usage_and_remaining
• sessions.quota.used_and_remaining_change
• sessions.quota.usage_label

Currently en.json contains 575 keys while ja.json contains 572, so these strings fall back to English in the Japanese UI.

Please also rerun the locale key parity check after updating. Thanks!