feat(i18n): add Japanese language support with translations and flag

[jp-yendo]

Description

What

Adds Japanese (日本語, ja) as a new built-in language for the Plik webapp.

Why

Expanding language coverage. Japanese is widely used in Asia (~125M speakers) and there is active demand from Japanese users in our environment.

Changes

• New files
  • webapp/src/locales/ja.json — Full Japanese translation (all keys from en.json)
  • webapp/public/flags/ja.svg — Flag of Japan (日の丸)
• Registration
  • webapp/src/settings.js — added ja entry to BUILTIN_LANGUAGES (alphabetical order, between it and nl)
  • webapp/src/i18n.js — added ja import and registration in messages
• Cross-locale — "ja": "日本語" added to the languagePicker section

Translation choices

Concept	Japanese	Notes
upload	アップロード	Standard katakana loanword
download	ダウンロード	Standard katakana loanword
passphrase	パスフレーズ	Katakana — distinct from password
password	パスワード	Universally understood katakana loanword
token	トークン	Standard katakana transliteration
admin	管理者	Native Japanese for "administrator"
one-shot	ワンショット	Katakana, consistent with other badges
Plurals	Single form	Japanese has no grammatical plural — "{n} day | {n} days" collapses to "{n} 日"

Notes

• Placeholder tokens ({value}, {count}, {n}, etc.) are preserved verbatim in all strings.
• JSON syntax validated.

Testing

Verified manually in the browser with both Japanese and English locales:

• Frontend builds cleanly
• Language picker shows 日本語 with the Japan flag
• Switching to Japanese updates all visible UI strings (upload view, download view, account, admin)
• Switching back to English continues to render correctly (no regressions)
• No fallback-to-English warnings in the console