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>

options/locale/locale_en-US.json

@@ -1157,7 +1157,6 @@
   "repo.watch": "Watch",
   "repo.watching": "Watching",
   "repo.ignoring": "Ignoring",
-  "repo.watch.options.required": "Select at least one event type.",
   "repo.watch.options.issues": "Issues",
   "repo.watch.options.pull_requests": "Pull Requests",
   "repo.watch.options.releases": "Releases",
@@ -1168,7 +1167,7 @@
   "repo.watch.mode.ignore": "Ignore",
   "repo.watch.mode.ignore.desc": "Never receive notifications from this repository.",
   "repo.watch.mode.custom": "Custom",
-  "repo.watch.mode.custom.desc": "Choose the events you want to receive notifications for.",
+  "repo.watch.mode.custom.desc": "Select events you want to be notified of, in addition to participating and mentions.",
   "repo.unstar": "Unstar",
   "repo.star": "Star",
   "repo.fork": "Fork",