fix: Fix issue target branch selection for non-collaborators (#36916)

This PR fixes a bug in the UI that prevented non-collaborator users (the
issue poster or creator) from setting the target branch (ref) of an
issue. The backend API already supports this, but the UI was rigidly
disabling the dropdown based only on collaborator status.

Changes:
- Enable the branch selector for the issue poster and during new issue
creation.
- Fix a typo (.IsIssueWriter -> .IsIssuePoster) that was preventing the
reference update URL from being correctly set for posters.

routers/web/repo/issue_new.go

@@ -146,6 +146,7 @@ func NewIssue(ctx *context.Context) {
 	}
 
 	ctx.Data["HasIssuesOrPullsWritePermission"] = ctx.Repo.Permission.CanWrite(unit.TypeIssues)
+	ctx.Data["IsIssuePoster"] = true // the current user will be the poster of the new issue
 
 	if !issueConfig.BlankIssuesEnabled && hasTemplates && !templateLoaded {
 		// The "issues/new" and "issues/new/choose" share the same query parameters "project" and "milestone", if blank issues are disabled, just redirect to the "issues/choose" page with these parameters.