Fix invalid sort field error on package artifact listing

The artifacts endpoint only supports sorting by: created_at, size, original_name
But the frontend was defaulting to 'name' (from the old tags endpoint).

- Change default sort from 'name' to 'created_at'
- Change default order from 'asc' to 'desc' (newest first)
- Remove sortable flag from version/tags columns (not DB fields)
- Add sortable flag to original_name and size columns

frontend/src/pages/PackagePage.tsx

@@ -112,10 +112,11 @@ function PackagePage() {
   const isSystemProject = projectName?.startsWith('_') ?? false;
 
   // Get params from URL
+  // Valid sort fields for artifacts: created_at, size, original_name
   const page = parseInt(searchParams.get('page') || '1', 10);
   const search = searchParams.get('search') || '';
-  const sort = searchParams.get('sort') || 'name';
-  const order = (searchParams.get('order') || 'asc') as 'asc' | 'desc';
+  const sort = searchParams.get('sort') || 'created_at';
+  const order = (searchParams.get('order') || 'desc') as 'asc' | 'desc';
 
   const updateParams = useCallback(
     (updates: Record<string, string | undefined>) => {
@@ -357,7 +358,7 @@ function PackagePage() {
         {
           key: 'version',
           header: 'Version',
-          sortable: true,
+          // version is from format_metadata, not a sortable DB field
           render: (a: PackageArtifact) => (
             <strong
               className={`tag-name-link ${selectedArtifact?.id === a.id ? 'selected' : ''}`}
@@ -371,6 +372,7 @@ function PackagePage() {
         {
           key: 'original_name',
           header: 'Filename',
+          sortable: true,
           className: 'cell-truncate',
           render: (a: PackageArtifact) => (
             <span title={a.original_name || a.id}>{a.original_name || a.id.slice(0, 12)}</span>
@@ -379,6 +381,7 @@ function PackagePage() {
         {
           key: 'size',
           header: 'Size',
+          sortable: true,
           render: (a: PackageArtifact) => <span>{formatBytes(a.size)}</span>,
         },
         {
@@ -423,10 +426,11 @@ function PackagePage() {
       ]
     : [
         // Regular project columns: Tag, Version, Filename, Size, Created
+        // Valid sort fields: created_at, size, original_name
         {
           key: 'tags',
           header: 'Tag',
-          sortable: true,
+          // tags is not a sortable DB field
           render: (a: PackageArtifact) => (
             <strong
               className={`tag-name-link ${selectedArtifact?.id === a.id ? 'selected' : ''}`}
@@ -440,6 +444,7 @@ function PackagePage() {
         {
           key: 'version',
           header: 'Version',
+          // version is from format_metadata, not a sortable DB field
           render: (a: PackageArtifact) => (
             <span className="version-badge">{getArtifactVersion(a) || '—'}</span>
           ),
@@ -447,6 +452,7 @@ function PackagePage() {
         {
           key: 'original_name',
           header: 'Filename',
+          sortable: true,
           className: 'cell-truncate',
           render: (a: PackageArtifact) => (
             <span title={a.original_name || undefined}>{a.original_name || '—'}</span>
@@ -455,6 +461,7 @@ function PackagePage() {
         {
           key: 'size',
           header: 'Size',
+          sortable: true,
           render: (a: PackageArtifact) => <span>{formatBytes(a.size)}</span>,
         },
         {