perf(grid): pre-stringify JSON/JSONB values and truncate cell display

Serialize JSON/JSONB as strings in Postgres and MySQL drivers to avoid
repeated JSON.stringify in the render loop. Truncate cell display to
256 chars so the browser doesn't lay out huge text nodes on scroll.

src/components/grid/DataGrid.vue

@@ -470,11 +470,13 @@ function toggleSort(colName: string) {
   }
 }
 
+const CELL_DISPLAY_MAX_LENGTH = 256;
+
 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);
+  const s = typeof value === "object" ? JSON.stringify(value) : String(value);
+  return s.length > CELL_DISPLAY_MAX_LENGTH ? s.slice(0, CELL_DISPLAY_MAX_LENGTH) : s;
 }
 
 function quoteIdent(name: string): string {