Localize user-facing error messages; move developer details to the log

[dadachi]

Summary

User-facing CodedError text was developer text in English (NativeAppTemplateAPIError::NoData, <message> [Status: 500], EntityAdapterError::InvalidOrMissingAttributes). It is now localized (en + ja), worded the same as the paid template and MyTurnTag iOS/Android; developer details go to the log.

Error	Shown (en)	Shown (ja)
NATIVEAPPTEMPLATE-2001 requestFailed without error_message	Something went wrong while communicating with the server. Please try again later.	サーバーとの通信中に問題が発生しました。しばらくしてからもう一度お試しください。
2001 with error_message	the API's message as is, without [Status: nnn]	(already localized by the API)
1001, 2002–2005, EntityAdapterError 2006–2008, 4001–4004	An unexpected error occurred.	予期しないエラーが発生しました。

• Codes and the [CODE] description format are unchanged; no flow changes. The generic strings contain no rename-target nouns.
• New CodedError.debugDescription keeps status code, API message, field name, underlying error; Error.debugCodedDescription returns it. All Failure logs use it, and URLSession.apiResponse logs failed and unreadable responses (Failure.request).
• Sign-up showed "UNKNOWN" for a failure without error_message; it now shows the coded generic text.
• AGENTS.md: user-facing error text is localized; developer details go to logs; NATIVEAPPTEMPLATE-nnnn codes stay untranslated. CHANGELOG updated.

Verification

• Failing tests first: 40 issues red before the change, green after.
• make lint: 0 violations, 0 files need formatting.
• Full suite on the iPhone: 434 passed (+1 known issue); 6 of them are new.
• xcrun xcstringstool sync against the build's .stringsdata: both new keys present, every key has ja translated.
• On device in Japanese (-testLanguage ja), a temporary uncommitted test against the local nativeapptemplateapi and a local HTTP server, checking Message(error:).message:
  • wrong password (local API, 401) → [NATIVEAPPTEMPLATE-2001] メールアドレスまたはパスワードが正しくありません。もう一度お試しください。 (log has Status: 401)
  • missing shop (local API, 404) → [NATIVEAPPTEMPLATE-2001] 見つかりませんでした。
  • 500 without body → [NATIVEAPPTEMPLATE-2001] サーバーとの通信中に問題が発生しました。しばらくしてからもう一度お試しください。
  • two resources for a detail request → [NATIVEAPPTEMPLATE-2002] 予期しないエラーが発生しました。
  • Keychain item missing → [NATIVEAPPTEMPLATE-4002] 予期しないエラーが発生しました。
• App installed and launched on the iPhone with devicectl (Japanese); it reaches the local API.

🤖 Generated with Claude Code

https://claude.ai/code/session_01K5hZEFqXhBsUQwSWsgR6eH