fix(grid): display JSONB values as JSON string instead of [object Object]

Closes #100

src/components/grid/DataGrid.vue

@@ -608,6 +608,7 @@ async function applyWhereSearch() {
 function formatCell(value: CellValue): string {
   if (value === null) return "NULL";
   if (typeof value === "boolean") return value ? "true" : "false";
+  if (typeof value === "object") return JSON.stringify(value);
   return String(value);
 }