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/workspace/TabContent.tsx

@@ -0,0 +1,49 @@
+import { useAppStore } from "@/stores/app-store";
+import { WorkspacePanel } from "./WorkspacePanel";
+import { TableDataView } from "@/components/table-viewer/TableDataView";
+import { TableStructure } from "@/components/table-viewer/TableStructure";
+
+export function TabContent() {
+  const { tabs, activeTabId, updateTab } = useAppStore();
+  const activeTab = tabs.find((t) => t.id === activeTabId);
+
+  if (!activeTab) {
+    return (
+      <div className="flex h-full items-center justify-center text-sm text-muted-foreground">
+        Open a query tab or select a table to get started.
+      </div>
+    );
+  }
+
+  switch (activeTab.type) {
+    case "query":
+      return (
+        <WorkspacePanel
+          key={activeTab.id}
+          connectionId={activeTab.connectionId}
+          initialSql={activeTab.sql}
+          onSqlChange={(sql) => updateTab(activeTab.id, { sql })}
+        />
+      );
+    case "table":
+      return (
+        <TableDataView
+          key={activeTab.id}
+          connectionId={activeTab.connectionId}
+          schema={activeTab.schema!}
+          table={activeTab.table!}
+        />
+      );
+    case "structure":
+      return (
+        <TableStructure
+          key={activeTab.id}
+          connectionId={activeTab.connectionId}
+          schema={activeTab.schema!}
+          table={activeTab.table!}
+        />
+      );
+    default:
+      return null;
+  }
+}