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.

templates/repo/issue/branch_selector_field.tmpl

@@ -14,14 +14,15 @@ Still needs to figure out:
 * Is "GitHub-like development sidebar (`#31899`)" good enough (or better) for your usage?
 */}}
 {{if and (not .Issue.IsPull) (not .PageIsComparePull)}}
+{{$canChangeRef := or .IsIssuePoster .HasIssuesOrPullsWritePermission}}
 <input id="ref_selector" name="ref" type="hidden" value="{{.Reference}}">
-<div class="ui dropdown select-branch branch-selector-dropdown ellipsis-text-items {{if not .HasIssuesOrPullsWritePermission}}disabled{{end}}"
+<div class="ui dropdown select-branch branch-selector-dropdown ellipsis-text-items {{if not $canChangeRef}}disabled{{end}}"
 	data-no-results="{{ctx.Locale.Tr "no_results_found"}}"
-	{{if and .Issue (or .IsIssueWriter .HasIssuesOrPullsWritePermission)}}data-url-update-issueref="{{$.RepoLink}}/issues/{{.Issue.Index}}/ref"{{end}}
+	{{if and .Issue $canChangeRef}}data-url-update-issueref="{{$.RepoLink}}/issues/{{.Issue.Index}}/ref"{{end}}
 >
 	<div class="ui button branch-dropdown-button">
 		<span class="text-branch-name gt-ellipsis">{{if .Reference}}{{$.RefEndName}}{{else}}{{ctx.Locale.Tr "repo.issues.no_ref"}}{{end}}</span>
-		{{if .HasIssuesOrPullsWritePermission}}{{svg "octicon-triangle-down" 14 "dropdown icon"}}{{end}}
+		{{if $canChangeRef}}{{svg "octicon-triangle-down" 14 "dropdown icon"}}{{end}}
 	</div>
 	<div class="menu">
 		<div class="ui icon search input">