fix(server): normalize mail server name (#14564)

fix #14562 
fix #14226 
fix #14192


#### PR Dependency Tree


* **PR #14564** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* SMTP and fallback SMTP name now default to empty and will use the
system hostname when not set.
* HELO hostname resolution includes stricter normalization/validation
for more reliable mail handshakes.

* **Documentation**
* Updated admin and config descriptions to explain hostname/HELO
behavior and fallback.

* **Tests**
* Added tests covering hostname normalization and rejection of invalid
HELO values.

* **Chores**
  * Updated example env and ignore rules.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.docker/selfhost/schema.json

@@ -197,8 +197,8 @@
       "properties": {
         "SMTP.name": {
           "type": "string",
-          "description": "Name of the email server (e.g. your domain name)\n@default \"AFFiNE Server\"\n@environment `MAILER_SERVERNAME`",
-          "default": "AFFiNE Server"
+          "description": "Hostname used for SMTP HELO/EHLO (e.g. mail.example.com). Leave empty to use the system hostname.\n@default \"\"\n@environment `MAILER_SERVERNAME`",
+          "default": ""
         },
         "SMTP.host": {
           "type": "string",
@@ -237,8 +237,8 @@
         },
         "fallbackSMTP.name": {
           "type": "string",
-          "description": "Name of the fallback email server (e.g. your domain name)\n@default \"AFFiNE Server\"",
-          "default": "AFFiNE Server"
+          "description": "Hostname used for fallback SMTP HELO/EHLO (e.g. mail.example.com). Leave empty to use the system hostname.\n@default \"\"",
+          "default": ""
         },
         "fallbackSMTP.host": {
           "type": "string",