Package weight, payload capacity and per-kg payout (ISS-045)

Packages carry a real weightKg (unit weight x count with +/-30% jitter),
plane capacity becomes maxPayloadKg instead of 6 slots, load validation
sums cargo weight against the payload, and the payout gains a per-kg part.
Loadout display shows tonnage; OpenAPI swaps CatalogEntry.capacity for
maxPayloadKg and adds PackageState.weightKg.

e2e/state.spec.ts

@@ -36,7 +36,7 @@ test('GET /api/state returns a seeded snapshot', async ({ page }) => {
 
   expect(Array.isArray(body.packages)).toBe(true);
   expect(body.packages.length).toBeGreaterThan(0);
-  for (const key of ['id', 'originIcao', 'destIcao', 'reward', 'cargoType', 'deadlineSimTime', 'status']) {
+  for (const key of ['id', 'originIcao', 'destIcao', 'reward', 'cargoType', 'weightKg', 'deadlineSimTime', 'status']) {
     expect(body.packages[0], `package.${key}`).toHaveProperty(key);
   }
 });