feat: auto-chart detection in SQL Explorer

Adds an '✨ Auto' button that analyzes query results and picks the
best chart type and column mapping:

- Date/time column + numeric → line chart (time series)
- Categorical + numeric → bar chart (categories)
- Two numeric columns → scatter plot
- Shows detected type and column names as a label

Click Auto, run any query, and it figures out the rest.

frontend/src/pages/SqlExplorer.tsx

@@ -29,7 +29,7 @@ interface SchemaInfo {
   tables: Array<{ name: string; columns: Array<{ name: string; type: string; nullable?: boolean }> }>;
 }
 
-type ChartType = 'none' | 'bar' | 'line' | 'scatter';
+type ChartType = 'none' | 'auto' | 'bar' | 'line' | 'scatter';
 
 export function SqlExplorerPage() {
   const qc = useQueryClient();
@@ -85,14 +85,74 @@ export function SqlExplorerPage() {
   }, []);
 
   // Build chart data from result
+  // Auto-detect best chart type and columns when chartType === 'auto'
+  const autoChart = (() => {
+    if (!result || result.columns.length < 2 || result.row_count === 0) {
+      return { type: 'bar' as const, x: 0, y: 1 };
+    }
+
+    const cols = result.columns;
+    const rows = result.rows;
+
+    // Classify each column as numeric or categorical
+    const isNumeric = cols.map((_, ci) => {
+      const sample = rows.slice(0, 10);
+      const numCount = sample.filter((r) => {
+        const v = r[ci];
+        return v != null && !isNaN(parseFloat(String(v))) && String(v).trim() !== '';
+      }).length;
+      return numCount > sample.length * 0.7;
+    });
+
+    // Check if a column looks like a date/time
+    const isDateLike = cols.map((c, ci) => {
+      const name = c.name.toLowerCase();
+      if (name.includes('date') || name.includes('time') || name.includes('_at') || name.includes('created') || name.includes('generated')) return true;
+      const sample = String(rows[0]?.[ci] ?? '');
+      return /^\d{4}-\d{2}/.test(sample);
+    });
+
+    const numericCols = cols.map((_, i) => i).filter((i) => isNumeric[i]);
+    const categoricalCols = cols.map((_, i) => i).filter((i) => !isNumeric[i]);
+    const dateCols = cols.map((_, i) => i).filter((i) => isDateLike[i]);
+
+    // Heuristics:
+    // 1. Date column + numeric → line chart (time series)
+    // 2. Categorical + numeric → bar chart (categories)
+    // 3. Two numeric columns → scatter plot
+    // 4. Fallback → bar chart with first categorical as X, first numeric as Y
+
+    if (dateCols.length > 0 && numericCols.length > 0) {
+      const xIdx = dateCols[0];
+      const yIdx = numericCols.find((i) => i !== xIdx) ?? numericCols[0];
+      return { type: 'line' as const, x: xIdx, y: yIdx };
+    }
+
+    if (categoricalCols.length > 0 && numericCols.length > 0) {
+      return { type: 'bar' as const, x: categoricalCols[0], y: numericCols[0] };
+    }
+
+    if (numericCols.length >= 2) {
+      return { type: 'scatter' as const, x: numericCols[0], y: numericCols[1] };
+    }
+
+    // Fallback: first col as X, second as Y, bar chart
+    return { type: 'bar' as const, x: 0, y: Math.min(1, cols.length - 1) };
+  })();
+
+  // Resolve effective chart settings (auto overrides manual selections)
+  const effectiveChartType = chartType === 'auto' ? autoChart.type : chartType;
+  const effectiveXCol = chartType === 'auto' ? autoChart.x : xCol;
+  const effectiveYCol = chartType === 'auto' ? autoChart.y : yCol;
+
   const chartData = result && result.columns.length >= 2
     ? result.rows.map((row) => {
-        const yRaw = row[yCol];
+        const yRaw = row[effectiveYCol];
         const yNum = yRaw != null ? parseFloat(String(yRaw)) : NaN;
         return {
-          x: row[xCol],
+          x: row[effectiveXCol],
           y: isNaN(yNum) ? 0 : yNum,
-          label: String(row[xCol] ?? ''),
+          label: String(row[effectiveXCol] ?? ''),
         };
       })
     : [];
@@ -249,16 +309,16 @@ export function SqlExplorerPage() {
           <Card className="shrink-0">
             <div className="mb-2 flex items-center gap-3">
               <span className="text-xs text-gray-500">Chart:</span>
-              {(['none', 'bar', 'line', 'scatter'] as ChartType[]).map((ct) => (
+              {(['none', 'auto', 'bar', 'line', 'scatter'] as ChartType[]).map((ct) => (
                 <button
                   key={ct}
                   onClick={() => setChartType(ct)}
                   className={`rounded px-2 py-0.5 text-xs ${chartType === ct ? 'bg-brand-600 text-white' : 'text-gray-400 hover:bg-surface-800'}`}
                 >
-                  {ct === 'none' ? 'Off' : ct}
+                  {ct === 'none' ? 'Off' : ct === 'auto' ? '✨ Auto' : ct}
                 </button>
               ))}
-              {chartType !== 'none' && (
+              {chartType !== 'none' && chartType !== 'auto' && (
                 <>
                   <select value={xCol} onChange={(e) => setXCol(Number(e.target.value))} className="rounded border border-surface-700 bg-surface-900 px-1 py-0.5 text-xs text-gray-300" aria-label="X axis column">
                     {result.columns.map((c, i) => <option key={i} value={i}>{c.name}</option>)}
@@ -269,10 +329,15 @@ export function SqlExplorerPage() {
                   </select>
                 </>
               )}
+              {chartType === 'auto' && result && (
+                <span className="text-[10px] text-gray-500">
+                  {autoChart.type} · X: {result.columns[autoChart.x]?.name} · Y: {result.columns[autoChart.y]?.name}
+                </span>
+              )}
             </div>
             {chartType !== 'none' && chartData.length > 0 && (
               <ResponsiveContainer width="100%" height={250}>
-                {chartType === 'bar' ? (
+                {effectiveChartType === 'bar' ? (
                   <BarChart data={chartData}>
                     <CartesianGrid strokeDasharray="3 3" stroke="#334155" />
                     <XAxis dataKey="label" tick={{ fill: '#6b7280', fontSize: 10 }} />
@@ -280,7 +345,7 @@ export function SqlExplorerPage() {
                     <Tooltip contentStyle={{ backgroundColor: '#1e293b', border: '1px solid #334155', borderRadius: 8 }} />
                     <Bar dataKey="y" fill="#3b82f6" />
                   </BarChart>
-                ) : chartType === 'line' ? (
+                ) : effectiveChartType === 'line' ? (
                   <LineChart data={chartData}>
                     <CartesianGrid strokeDasharray="3 3" stroke="#334155" />
                     <XAxis dataKey="label" tick={{ fill: '#6b7280', fontSize: 10 }} />
@@ -291,8 +356,8 @@ export function SqlExplorerPage() {
                 ) : (
                   <ScatterChart>
                     <CartesianGrid strokeDasharray="3 3" stroke="#334155" />
-                    <XAxis dataKey="x" tick={{ fill: '#6b7280', fontSize: 10 }} name={result.columns[xCol]?.name} />
+                    <XAxis dataKey="x" tick={{ fill: '#6b7280', fontSize: 10 }} name={result.columns[effectiveXCol]?.name} />
-                    <YAxis dataKey="y" tick={{ fill: '#6b7280', fontSize: 10 }} name={result.columns[yCol]?.name} />
+                    <YAxis dataKey="y" tick={{ fill: '#6b7280', fontSize: 10 }} name={result.columns[effectiveYCol]?.name} />
                     <Tooltip contentStyle={{ backgroundColor: '#1e293b', border: '1px solid #334155', borderRadius: 8 }} />
                     <Scatter data={chartData} fill="#3b82f6" />
                   </ScatterChart>