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>

tests/db.test.js

@@ -2,7 +2,7 @@ import { describe, it, expect, beforeEach } from 'vitest'
 import {
   _resetForTest,
   getInstances, getInstance, getDistinctStacks,
-  createInstance, updateInstance, deleteInstance,
+  createInstance, updateInstance, deleteInstance, importInstances,
 } from '../server/db.js'
 
 beforeEach(() => _resetForTest());
@@ -166,6 +166,25 @@ describe('deleteInstance', () => {
   });
 });
 
+// ── importInstances ───────────────────────────────────────────────────────────
+
+describe('importInstances', () => {
+  const base = { state: 'deployed', stack: 'production', atlas: 0, argus: 0, semaphore: 0, patchmon: 0, tailscale: 0, andromeda: 0, tailscale_ip: '', hardware_acceleration: 0 };
+
+  it('replaces all existing instances with the imported set', () => {
+    createInstance({ ...base, name: 'old', vmid: 1 });
+    importInstances([{ ...base, name: 'new', vmid: 2 }]);
+    expect(getInstance(1)).toBeNull();
+    expect(getInstance(2)).not.toBeNull();
+  });
+
+  it('clears all instances when passed an empty array', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    importInstances([]);
+    expect(getInstances()).toEqual([]);
+  });
+});
+
 // ── Test environment boot isolation ───────────────────────────────────────────
 
 describe('test environment boot isolation', () => {