fix(launches): cancel mission flights on exit and name a failed index load

Review fixes on top of #1578:
- Exiting Space Missions cancels the layer's own camera flights again (entry,
  roster preview, mission focus); only a flight navigation started after
  releaseCameraOwnership() runs on.
- A failed first index load says so: "Space Missions unavailable: the
  mission index could not load. Try again later." on both the Context and
  Data Layers routes, instead of a generic transition toast. Translated.
- Regression tests for both; docs and changelog describe the real feedback.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/launches/policyHelpers.js

@@ -229,6 +229,7 @@ export function createPolicyHelpers({
       layerState._viewer.camera.positionCartographic?.height,
     );
     const position = Cesium.Cartesian3.fromDegrees(launch.lon, launch.lat);
+    layerState._cameraReleased = false;
     layerState._viewer.camera.flyToBoundingSphere(
       new Cesium.BoundingSphere(position, 0),
       {