enhance: allow custom watches to bypass on-mention email restriction (#39652)

Users with "Only Email on Mention" can now still receive email for the
events they picked in a custom repository watch, covering issues, pull
requests and releases. The setting is renamed to "Only Email on Mention
and Custom Watches" to make this explicit, matching GitHub's
"Participating, @mentions and custom".

"All activity" watches don't bypass the setting, because they are also
created automatically by `AUTO_WATCH_NEW_REPOS` for repository creators
and team members. To keep picking every event distinct from "All
activity", the custom watch dialog now stores a new `WatchModeCustom`.
Existing custom watches keep working without a migration.

## Screenshots
### Before
<img width="1650" height="675" alt="grafik"
src="https://github.com/user-attachments/assets/ecc56e20-cd05-41e9-a840-607b352c50a8"
/>

### After
<img width="1657" height="671" alt="grafik"
src="https://github.com/user-attachments/assets/a406db94-3ac4-4587-b840-0cd472b135c5"
/>


Addresses https://github.com/openSUSE/openSUSE-git/issues/183
Ref: https://github.com/go-gitea/gitea/issues/35492

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: Giteabot <teabot@gitea.io>

options/locale/locale_en-US.json

@@ -929,7 +929,7 @@
   "settings.delete_account_title": "Delete User Account",
   "settings.delete_account_desc": "Are you sure you want to permanently delete this user account?",
   "settings.email_notifications.enable": "Enable Email Notifications",
-  "settings.email_notifications.onmention": "Only Email on Mention",
+  "settings.email_notifications.onmention_and_custom": "Only Email on Mention and Custom Watches",
   "settings.email_notifications.disable": "Disable Email Notifications",
   "settings.email_notifications.submit": "Set Email Preference",
   "settings.email_notifications.andyourown": "And Your Own Notifications",