Update the list of watchers and stargazers when clicking watch/unwatch or star/unstar (#32570)

We make sure the user cards are updated

- Fixes https://github.com/go-gitea/gitea/issues/32561

I also removed `ctx.Data["PageIsWatchers"] = true` and
`ctx.Data["PageIsStargazers"] = true` as they are not used anywhere.

# Before

![before](https://github.com/user-attachments/assets/e3bc3235-35eb-4eda-862d-bdf2510282ea)

# After

![after](https://github.com/user-attachments/assets/bc0488a5-8399-4cf6-95c9-17328a9702eb)

---------

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

routers/web/repo/repo.go

@@ -352,6 +352,9 @@ func Action(ctx *context.Context) {
 		ctx.Data["IsStaringRepo"] = repo_model.IsStaring(ctx, ctx.Doer.ID, ctx.Repo.Repository.ID)
 	}
 
+	// see the `hx-trigger="refreshUserCards ..."` comments in tmpl
+	ctx.RespHeader().Add("hx-trigger", "refreshUserCards")
+
 	switch ctx.PathParam(":action") {
 	case "watch", "unwatch", "star", "unstar":
 		// we have to reload the repository because NumStars or NumWatching (used in the templates) has just changed