feat(i18n): add Japanese (ja-JP) locale (THU-MAIC#365)

- Add complete ja-JP.json with all 785 translation keys

- Register ja-JP in supportedLocales (locales.ts)

- All keys verified: 0 missing, 0 extra, 0 emoji drift

Author: YizukiAme

lib/i18n/locales.ts

@@ -16,4 +16,5 @@ export type LocaleEntry = {
 export const supportedLocales = [
   { code: 'zh-CN', label: '简体中文', shortLabel: 'CN' },
   { code: 'en-US', label: 'English', shortLabel: 'EN' },
+  { code: 'ja-JP', label: '日本語', shortLabel: 'JA' },
 ] as const satisfies readonly LocaleEntry[];