Two-plane load fix + e2e coverage (ISS-055, v0.31.0)

- LoadControls preselects the first eligible plane so Load is one-click with N planes
- LeftPane queue testids scoped by plane id (queue-leg/cancel/add[-dest]-{plane.id})
  — unscoped ids duplicated in the DOM with two on-board blocks
- new POST /api/test/add-cash hook (FLIGHT_TEST_HOOKS=1, default €1M) so e2e can
  afford a second plane; returns the full state snapshot
- new serial spec e2e/two-planes.spec.ts: load, parallel fly + ISS-057 slow/resume,
  refuel, relocate, per-plane queues with auto-chain, sell (6 tests)
- e2e/queue.spec.ts locators updated to the scoped testids

package.json

@@ -1,6 +1,6 @@
 {
   "name": "flight-ops",
-  "version": "0.30.0",
+  "version": "0.31.0",
   "private": true,
   "scripts": {
     "dev": "next dev -p 3000",