fix(imap): fix mojibake in nested emails, empty headers, and date handling (#909)

* fix(imap): fix mojibake in nested emails, empty headers, and date handling

- Add line-by-line mojibake fix fallback for complex emails with mixed content
- Apply empty header cleanup globally to fix nested message/rfc822 parts
- Add locale-independent date formatting (format_imap_date, format_rfc2822_date)
- Fill missing Date header from created_at field
- Fix getSubPart for non-multipart messages
- Accept CREATE requests from clients (e.g. Gmail creating Drafts)
- Strip whitespace from IMAP password
- Use MIMEText instead of MIMEMultipart for sent mail generation
- Keep body in original CTE encoding for correct BODYSTRUCTURE
- Update CHANGELOG (zh/en)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: consolidate IMAP changelog entries into single line

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -18,6 +18,7 @@
 
 - fix: |自动回复| 修复 `source_prefix` 为空字符串时自动回复不触发的问题（#459），空值现在正确匹配所有发件人
 - fix: |OAuth2| 修复 Android via 浏览器等移动端 OAuth2 登录时 sessionStorage 丢失导致回调失败的问题，新增 localStorage 兜底（#900）
+- fix: |IMAP| 修复嵌套回复邮件乱码、Gmail 空 Content-Type 头解析失败、缺失 Date 头及 locale 依赖日期格式等问题
 
 ### Testing