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>

models/issues/issue_search.go

@@ -477,7 +477,7 @@ func applySubscribedCondition(sess db.Session, subscriberID int64) {
 				Select("repo_id").
 				From("watch").
 				Where(builder.And(builder.Eq{"user_id": subscriberID},
-					builder.In("mode", repo_model.WatchModeNormal, repo_model.WatchModeAuto))),
+					builder.In("mode", repo_model.WatchModeNormal, repo_model.WatchModeAuto, repo_model.WatchModeCustom))),
 			),
 		),
 	)