feat: settings modal with database export and import

Adds a gear button to the nav that opens a settings modal with:
- Export: GET /api/export returns all instances as a JSON backup file
  with a Content-Disposition attachment header
- Import: POST /api/import validates and bulk-replaces all instances;
  client uses FileReader to POST the parsed JSON, with a confirm dialog
  before destructive replace

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

server/routes.js

@@ -1,7 +1,7 @@
 import { Router } from 'express';
 import {
   getInstances, getInstance, getDistinctStacks,
-  createInstance, updateInstance, deleteInstance,
+  createInstance, updateInstance, deleteInstance, importInstances,
 } from './db.js';
 
 export const router = Router();
@@ -104,6 +104,35 @@ router.put('/instances/:vmid', (req, res) => {
   }
 });
 
+// GET /api/export
+router.get('/export', (_req, res) => {
+  const instances = getInstances();
+  const date = new Date().toISOString().slice(0, 10);
+  res.setHeader('Content-Disposition', `attachment; filename="catalyst-backup-${date}.json"`);
+  res.json({ version: 1, exported_at: new Date().toISOString(), instances });
+});
+
+// POST /api/import
+router.post('/import', (req, res) => {
+  const { instances } = req.body ?? {};
+  if (!Array.isArray(instances)) {
+    return res.status(400).json({ error: 'body must contain an instances array' });
+  }
+  const errors = [];
+  for (const [i, row] of instances.entries()) {
+    const errs = validate(normalise(row));
+    if (errs.length) errors.push({ index: i, errors: errs });
+  }
+  if (errors.length) return res.status(400).json({ errors });
+  try {
+    importInstances(instances.map(normalise));
+    res.json({ imported: instances.length });
+  } catch (e) {
+    console.error('POST /api/import', e);
+    res.status(500).json({ error: 'internal server error' });
+  }
+});
+
 // DELETE /api/instances/:vmid
 router.delete('/instances/:vmid', (req, res) => {
   const vmid = parseInt(req.params.vmid, 10);