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/api.test.js

@@ -239,6 +239,52 @@ describe('DELETE /api/instances/:vmid', () => {
   })
 })
 
+// ── GET /api/export ───────────────────────────────────────────────────────────
+
+describe('GET /api/export', () => {
+  it('returns 200 with instances array and attachment header', async () => {
+    await request(app).post('/api/instances').send(base)
+    const res = await request(app).get('/api/export')
+    expect(res.status).toBe(200)
+    expect(res.headers['content-disposition']).toMatch(/attachment/)
+    expect(res.body.instances).toHaveLength(1)
+    expect(res.body.instances[0].name).toBe('traefik')
+  })
+
+  it('returns empty instances array when no data', async () => {
+    const res = await request(app).get('/api/export')
+    expect(res.body.instances).toEqual([])
+  })
+})
+
+// ── POST /api/import ──────────────────────────────────────────────────────────
+
+describe('POST /api/import', () => {
+  it('replaces all instances and returns imported count', async () => {
+    await request(app).post('/api/instances').send(base)
+    const res = await request(app).post('/api/import')
+      .send({ instances: [{ ...base, vmid: 999, name: 'imported' }] })
+    expect(res.status).toBe(200)
+    expect(res.body.imported).toBe(1)
+    expect((await request(app).get('/api/instances')).body[0].name).toBe('imported')
+  })
+
+  it('returns 400 if instances is not an array', async () => {
+    expect((await request(app).post('/api/import').send({ instances: 'bad' })).status).toBe(400)
+  })
+
+  it('returns 400 with per-row errors for invalid rows', async () => {
+    const res = await request(app).post('/api/import')
+      .send({ instances: [{ ...base, name: '', vmid: 1 }] })
+    expect(res.status).toBe(400)
+    expect(res.body.errors[0].index).toBe(0)
+  })
+
+  it('returns 400 if body has no instances key', async () => {
+    expect((await request(app).post('/api/import').send({})).status).toBe(400)
+  })
+})
+
 // ── Static assets & SPA routing ───────────────────────────────────────────────
 
 describe('static assets and SPA routing', () => {