fix: remove sticky thead — overlapped table toolbar (#4)

The thead had position: sticky; top: var(--topbar-height), pinning it
to 64px from the viewport. Inside the new .table-card with a toolbar
above, this pulled the header up out of its natural flow and overlapped
the toolbar and first data row. Let thead flow normally — design
doesn't require sticky behavior.

public/css/shared.css

@@ -318,12 +318,6 @@ table {
     font-size: 14px;
 }
 
-thead {
-    position: sticky;
-    top: var(--topbar-height);
-    z-index: 10;
-}
-
 th {
     padding: 0 16px;
     height: 48px;