Feat/locale ja

[Zulfazli (fazelllyyy)]

Summary

Adds full Japanese (ja) support for the frontend i18n system, following the same pattern as #436 (Hindi) and #463 (Indonesian). Related to #337 (International Language Support).

Changes

• New locale bundle src/i18n/locales/ja/ — all 10 domains translated (common, upload, chart, model, encoding, messages, navigation, dataLoading, loader, errors) plus the index.ts aggregator
• Registered ja in src/i18n/locales/index.ts and in the resources map in src/i18n/index.ts (it flows automatically into SUPPORTED_UI_LANGUAGES)
• Extended tests/frontend/unit/app/i18nLocales.test.ts with a ja-vs-en key-parity test
• No changes needed in src/app/App.tsx (ja: '日本語' label already existed) or in the backend ("ja" already exists in LANGUAGE_DISPLAY_NAMES along with its polite-form extra rule, so agent language instructions work out of the box)

Translation notes

• Polite Japanese (です/ます体) throughout all user-facing strings, per the repo's rule for Japanese
• Translation keys untouched; all {{interpolation}} placeholders, _plural forms, and inline HTML tags (<refreshAction>, <strong>) preserved verbatim
• Untranslatable identifiers kept in English per TRANSLATION_GUIDE.md (chart types, channel keys, spec values); tooltips (encoding.channel*_tip, chart.templateNames.*) translated instead
• Commands, code snippets, paths, and URLs in loader.json left literal
• chart.vegaLocale localized (Japanese day/month names, 午前/午後 periods); no code change needed in vega-locale.ts

Verification

• JSON validity: all 10 files parse cleanly
• Key parity across bundles (same spread-merge semantics as index.ts): en/hi/zh/id/ja each resolve to exactly 1860 keys — 0 missing, 0 extra, 0 placeholder/tag mismatches
• New unit test asserts ja key alignment with en
• Note: a full vitest/tsc run was not possible in this environment (no node_modules available); please rely on CI