feat(webui): add Japanese (ja) localization

[honbou]

Summary

Adds Japanese localization to the PicoClaw WebUI, as requested in #3272.

• Add web/frontend/src/i18n/locales/ja.json — full Japanese translation of en.json (all 968 lines)
• Register the ja resource and dayjs/locale/ja in i18n/index.ts, including the languageChanged dayjs locale switch
• Add a 日本語 entry to the language switcher dropdown in app-header.tsx

Verification

• Key structure and {{placeholder}} interpolation variables programmatically verified to match en.json exactly (no missing / extra keys, no placeholder mismatches)
• Built the frontend and picoclaw-launcher (linux/amd64) and confirmed the UI renders in Japanese via the language switcher and browser auto-detection

Closes #3272

🤖 Generated with Claude Code

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