fix: enabled toggle — list-row click + edit-form save

Two bugs in the channel-enabled affordance:

1. List-row toggle was a static span with no handler; the row's
   row-link overlay swallowed every click and routed to /edit. Add
   POST /settings/notifications/{id}/toggle backed by a new store
   method SetNotificationChannelEnabled, and turn the row toggle
   into an htmx-driven button that swaps in the new state. Use
   event.stopPropagation() on the toggle so it beats the row link.

2. Edit-form toggle visually flipped but the underlying checkbox
   reverted: the visual span lives inside the <label>, so clicking
   it fired the inline JS handler AND the label's native
   checkbox-toggle, cancelling out. Bind to the checkbox 'change'
   event instead and let the label do the toggling — the JS just
   mirrors check.checked into the .on class.

internal/server/http/server.go

@@ -325,6 +325,7 @@ func (s *Server) routes(r chi.Router) {
 		r.Get("/settings/notifications/{id}/edit", s.handleUINotificationEditGet)
 		r.Post("/settings/notifications/{id}/edit", s.handleUINotificationEditPost)
 		r.Post("/settings/notifications/{id}/delete", s.handleUINotificationDelete)
+		r.Post("/settings/notifications/{id}/toggle", s.handleUINotificationToggle)
 	}
 
 	// Browser job-log stream (separate from /ws/agent so the auth