fix: SQL Explorer chart parses string values as floats

The pg-query API returns all values as strings. The chart builder
was using Number() which returns NaN for non-numeric strings.
Now uses parseFloat with NaN fallback to 0.

frontend/src/pages/SqlExplorer.tsx

@@ -86,11 +86,15 @@ export function SqlExplorerPage() {
 
   // Build chart data from result
   const chartData = result && result.columns.length >= 2
-    ? result.rows.map((row) => ({
-        x: row[xCol],
-        y: Number(row[yCol]) || 0,
-        label: String(row[xCol]),
-      }))
+    ? result.rows.map((row) => {
+        const yRaw = row[yCol];
+        const yNum = yRaw != null ? parseFloat(String(yRaw)) : NaN;
+        return {
+          x: row[xCol],
+          y: isNaN(yNum) ? 0 : yNum,
+          label: String(row[xCol] ?? ''),
+        };
+      })
     : [];
 
   // Split saved queries into pre-built (no id-based delete) and user-saved