enhance: refine repo watching (#38835)

Follow-up to https://github.com/go-gitea/gitea/pull/37571.

"Participating and mentions" deleted the watch row, so choosing it
dropped you out of the watcher count. It is a watch like the others, so
it now keeps a row and simply subscribes to no events.

The dashboard feed ignored the per-event options, so a "Custom: issues"
watcher still got pull request activity there. It now gates on the same
options as mail and notifications. That also closes a gap where pull
request reviews bypassed the permission check.

Also, address
https://github.com/go-gitea/gitea/pull/37571#discussion_r3740487363 and
reword a UI text for clarity.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/context/repo.go

@@ -644,8 +644,7 @@ func repoAssignmentPrepareTemplateData(ctx *Context, data *repoAssignmentPrepare
 			ctx.ServerError("GetWatch", err)
 			return
 		}
-		ctx.Data["Watch"] = watch
-		ctx.Data["IsWatchingRepo"] = repo_model.IsWatchMode(watch.Mode)
+		ctx.Data["RepoWatch"] = watch
 		ctx.Data["IsStaringRepo"] = repo_model.IsStaring(ctx, ctx.Doer.ID, repo.ID)
 	}