feat(org): add pinned repositories management UI

Implements a settings page for organization admins to manage pinned repositories. Adds ability to pin/unpin repos, reorder them via drag-and-drop, and view currently pinned repos. Includes new routes, templates, locale strings, and helper function IsErrOrgPinnedRepoAlreadyExist for error handling.

models/organization/org_pinned.go

@@ -271,3 +271,9 @@ type ErrOrgPinnedRepoAlreadyExist struct {
 func (err ErrOrgPinnedRepoAlreadyExist) Error() string {
 	return "repository is already pinned"
 }
+
+// IsErrOrgPinnedRepoAlreadyExist checks if err is ErrOrgPinnedRepoAlreadyExist
+func IsErrOrgPinnedRepoAlreadyExist(err error) bool {
+	_, ok := err.(ErrOrgPinnedRepoAlreadyExist)
+	return ok
+}