feat: Phase 2 — Mission rotation management + multi-file upload

- Backend: add terrain field to Arma3MissionManager.list_missions()
- Backend: add missions field to ServerConfig Pydantic model
- Backend: add GET /missions/rotation and PUT /missions/rotation endpoints
- Frontend: Mission type gains terrain field; new MissionRotationEntry type
- Frontend: useServerMissionRotation and useUpdateMissionRotation hooks
- Frontend: useUploadMission updated to accept File[] with sequential upload
- Frontend: MissionList redesigned with Available Missions + Mission Rotation sections
- Frontend: per-file upload progress tracking, terrain badges, difficulty select
- Tests: 5 new tests; fixed existing useUploadMission test for File[] API; 141 pass

backend/adapters/arma3/mission_manager.py

@@ -52,10 +52,12 @@ class Arma3MissionManager:
         try:
             for entry in missions_dir.iterdir():
                 if entry.is_file() and entry.suffix.lower() == _ALLOWED_EXTENSION:
+                    parsed = self.parse_mission_filename(entry.name)
                     missions.append({
                         "name": entry.stem,
                         "filename": entry.name,
                         "size_bytes": entry.stat().st_size,
+                        "terrain": parsed["terrain"],
                     })
         except OSError as exc:
             raise AdapterError(f"Cannot list missions: {exc}") from exc