feat: add SQL editor, query results, and workspace panels

Add CodeMirror 6 SQL editor with Ctrl+Enter execution, ResultsTable
with virtual scrolling and resizable columns, ResultsPanel, EditableCell,
WorkspacePanel (editor + results split), and TabContent router.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/results/ResultsPanel.tsx

@@ -0,0 +1,69 @@
+import { ResultsTable } from "./ResultsTable";
+import type { QueryResult } from "@/types";
+import { Loader2, AlertCircle } from "lucide-react";
+
+interface Props {
+  result?: QueryResult | null;
+  error?: string | null;
+  isLoading?: boolean;
+  onCellDoubleClick?: (
+    rowIndex: number,
+    colIndex: number,
+    value: unknown
+  ) => void;
+  highlightedCells?: Set<string>;
+}
+
+export function ResultsPanel({
+  result,
+  error,
+  isLoading,
+  onCellDoubleClick,
+  highlightedCells,
+}: Props) {
+  if (isLoading) {
+    return (
+      <div className="flex h-full items-center justify-center text-muted-foreground">
+        <Loader2 className="mr-2 h-5 w-5 animate-spin" />
+        Executing query...
+      </div>
+    );
+  }
+
+  if (error) {
+    return (
+      <div className="flex h-full items-center justify-center p-4">
+        <div className="flex items-start gap-2 rounded-md border border-destructive/50 bg-destructive/10 p-3 text-sm text-destructive">
+          <AlertCircle className="mt-0.5 h-4 w-4 shrink-0" />
+          <pre className="whitespace-pre-wrap font-mono text-xs">{error}</pre>
+        </div>
+      </div>
+    );
+  }
+
+  if (!result) {
+    return (
+      <div className="flex h-full items-center justify-center text-sm text-muted-foreground">
+        Press Ctrl+Enter to execute query
+      </div>
+    );
+  }
+
+  if (result.columns.length === 0) {
+    return (
+      <div className="flex h-full items-center justify-center text-sm text-muted-foreground">
+        Query executed successfully. {result.row_count} rows affected.
+      </div>
+    );
+  }
+
+  return (
+    <ResultsTable
+      columns={result.columns}
+      types={result.types}
+      rows={result.rows}
+      onCellDoubleClick={onCellDoubleClick}
+      highlightedCells={highlightedCells}
+    />
+  );
+}