fix#13627
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## Summary by CodeRabbit
* **New Features**
* Added configurable display names for primary and fallback SMTP
servers, improving email sender identification.
* Defaults to “AFFiNE Server,” with support for MAILER_SERVERNAME
environment variable for the primary SMTP.
* Exposed in admin settings for easy setup alongside existing SMTP
options.
* Names are now passed through to mail transport options for consistent
use across emails.
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