feat: editor enhancements, DataGrid refactor, and sidebar improvements

- Add bracket auto-close and matching to SQL editor (closeBrackets, bracketMatching)
- Show truncation warning banner when query results exceed 10,000 rows
- Extract shared export format functions (CSV/JSON/SQL INSERT) to lib/exportFormats.ts
- Add progressive rendering to sidebar tree (50-node cap with "show more")
- Fix connection node expanding before connect succeeds
- Split DataGrid.vue (3177→2535 lines) into 4 composables:
  useDataGridExport, useDataGridColumnResize, useDataGridSelection, useDataGridEditor

src/lib/exportFormats.ts

@@ -0,0 +1,39 @@
+export type ExportCellValue = string | number | boolean | null;
+
+export function formatCsv(columns: string[], rows: ExportCellValue[][]): string {
+  const esc = (v: string) => `"${v.replace(/"/g, '""')}"`;
+  const header = columns.map(esc).join(",");
+  const body = rows.map((row) => row.map((c) => esc(c === null ? "" : String(c))).join(",")).join("\n");
+  return `${header}\n${body}`;
+}
+
+export function formatJson(columns: string[], rows: ExportCellValue[][]): string {
+  const data = rows.map((row) => {
+    const obj: Record<string, unknown> = {};
+    columns.forEach((col, i) => {
+      obj[col] = row[i];
+    });
+    return obj;
+  });
+  return JSON.stringify(data, null, 2);
+}
+
+export function formatSqlInsert(
+  qualifiedName: string,
+  columns: string[],
+  rows: ExportCellValue[][],
+  quoteIdent: (name: string) => string,
+): string {
+  const cols = columns.map((c) => quoteIdent(c)).join(", ");
+  const lines = rows.map((row) => {
+    const vals = row
+      .map((v) => {
+        if (v === null) return "NULL";
+        if (typeof v === "number" || typeof v === "boolean") return String(v);
+        return `'${String(v).replace(/'/g, "''")}'`;
+      })
+      .join(", ");
+    return `INSERT INTO ${qualifiedName} (${cols}) VALUES (${vals});`;
+  });
+  return lines.join("\n");
+}