[kongesque]

Summary

• Report when LINE approves a QR login but returns no usable Letter Sealing keys, instead of a generic local setup failure. The message warns that retrying can replace the session again and points to email login.
• Add a diagnostic for qrCodeLoginV2 metadata: metaData and each key field as absent, null, empty, or present, errorCode as absent, SUCCESS, or other, and a count of unrecognized fields. It holds fixed words and a count only, never server text.
• Reword the QR Letter Sealing requirement in all four READMEs, since the reporter has no such toggle, and add a troubleshooting note to all four CLI guides.

Encryption behavior is unchanged: the session is still not saved, NoE2EE is never inferred, and final login is never retried.

Refs #25. This does not fix the reported login failure. The root cause is unknown, so the issue should stay open until the new diagnostic and Chrome's behavior for a missing keyId are known.

Validation

• Passed on macOS arm64 with Go 1.27.3: gofmt, go vet, go build ./cmd/line, and go test -race ./internal/... ./cmd/line ./pkg/line/... ./pkg/e2ee.
• New tests cover nine metadata shapes, a check that server values never reach the error, and the login message with nothing saved.
• Not run locally: goimports, staticcheck, and the ./pkg and ./pkg/ltsm suites. No imports changed, and that code is untouched.
• Not tested against live LINE; all responses are synthetic. Translated text has not had native-speaker review.