Add sim_data_gen.py to forecast_app with generated JSON files

Created simulation-based data generator for forecast_app that:
- Uses supply_chain_sim.gen_data() to generate realistic data
- Exports data in the same format as data_generator.py
- Generates 3 JSON files with sim_ prefix:
  * sim_supply_chain_data_full.json (11 years)
  * sim_supply_chain_data_training.json (10 years)
  * sim_supply_chain_data_test.json (year 11)

The generated data is more realistic as it uses the complete
supply chain simulation with all entities and dynamics.