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/db.js

@@ -125,6 +125,21 @@ export function deleteInstance(vmid) {
   return db.prepare('DELETE FROM instances WHERE vmid = ?').run(vmid);
 }
 
+export function importInstances(rows) {
+  db.exec('BEGIN');
+  db.exec('DELETE FROM instances');
+  const insert = db.prepare(`
+    INSERT INTO instances
+      (name, state, stack, vmid, atlas, argus, semaphore, patchmon,
+       tailscale, andromeda, tailscale_ip, hardware_acceleration)
+    VALUES
+      (@name, @state, @stack, @vmid, @atlas, @argus, @semaphore, @patchmon,
+       @tailscale, @andromeda, @tailscale_ip, @hardware_acceleration)
+  `);
+  for (const row of rows) insert.run(row);
+  db.exec('COMMIT');
+}
+
 // ── Test helpers ──────────────────────────────────────────────────────────────
 
 export function _resetForTest() {