fix(grid): display JSONB values as JSON string in edit mode and detail panel (#152)

String() on objects produces [object Object]. Use JSON.stringify()
consistently wherever cell values are converted to strings.

src/lib/gridSelection.ts

@@ -51,6 +51,7 @@ export function extractSelection(
 function displayValue(value: GridCellValue): 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);
 }