[Bug] gmail send --reply-to sets wrong timezone (+0900) in Date header causing delivery failure

[kiagentkronos-cell]

Bug Description

When using gog gmail send --reply-to <messageId>, the resulting email has an incorrect timezone in the Date header (+0900, Tokyo timezone), which causes Google to reject the message with:

An error occurred. Your message was not sent.

Steps to Reproduce

1. Have an existing email thread with a message ID
2. Run: gog gmail send --to recipient@example.com --subject "Re: Subject" --body "..." --reply-to <messageId>
3. The sent email will have a Date header like: Tue, 24 Mar 2026 19:13:39 +0900 (Tokyo time) instead of the correct local timezone or UTC

Expected Behavior

The Date header should use the system timezone (or UTC) — not hardcoded +0900.

Actual Behavior

Email Date header is set to +0900 (Tokyo/JST), causing Google's delivery to fail with a generic error message.

Workaround

Do not use --reply-to flag. Send as a new email with a manually prefixed subject (e.g. Re: Original Subject). The email sends successfully without this flag.

Environment

• gog version: v0.12.0 (c18c58c 2026-03-09T05:53:14Z)
• OS: Linux (Ubuntu 24.04)
• Using OAuth credentials

Impact

The --reply-to flag is effectively broken — using it causes email delivery to silently fail with a non-descriptive error message.

[steipete]

Fixed on main in 2c48bd8, with lint cleanup in 6bdc215.

Outbound Gmail messages now build the RFC822 Date header using the configured timezone (GOG_TIMEZONE / default_timezone) before falling back to the host local timezone, so replies do not inherit an unexpected host-local offset.

Regression coverage added: TestBuildRFC822_DateHeaderUsesConfiguredTimezone.

Verified locally:

• go test ./internal/cmd -run 'TestBuildRFC822|TestGmailSend|TestGmailDraft' -count=1
• make ci