fix(frontend): valid scalar markup + selector/truncated coverage in ResultsPanel

- Replace invalid role="strong" with data-testid="scalar-value"
- Add test (d): selecting "tutti" re-fetches preview with the large limit
- Render "(risultati troncati)" indicator when truncated; assert in test (a)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/viewers/ResultsPanel.tsx

@@ -64,14 +64,17 @@ export function ResultsPanel({ sessionId }: Props) {
 
       {/* Scalar result: 1 col × 1 row → bold value */}
       {data && isScalar && (
-        <strong role="strong">{String(data.rows[0][0])}</strong>
+        <strong data-testid="scalar-value">{String(data.rows[0][0])}</strong>
       )}
 
       {/* Grid for everything else */}
       {data && !isScalar && (
-        <div className="ag-theme-alpine" style={{ height: 400, width: "100%" }}>
-          <AgGridReact columnDefs={columnDefs} rowData={rowData} />
-        </div>
+        <>
+          <div className="ag-theme-alpine" style={{ height: 400, width: "100%" }}>
+            <AgGridReact columnDefs={columnDefs} rowData={rowData} />
+          </div>
+          {data.truncated && <div>(risultati troncati)</div>}
+        </>
       )}
     </div>
   );