EntityFishMerchant

[Seggellion]

• [x] Create EntityFishMerchant class extending EntityMerchant.
• [x] Implement buying logic for fish commodities (common, rare, premium).
• [x] Update city inventory with food stock.
• [ ] Implement the basic UI for selling fish.
• [x] Focus on integrating with the CityInventory system to reflect stock updates.
• [ ] Keep NPC close to spawn location.
• [ ] Food value represented in inventory (different fish weight have different values)
• [ ] Fish price should be determined by weight times price per weight-unit.
• [ ] There should be a list of transactions so that we can determine who has sold the most fish, and we can have leaderboards.
• [ ] Make sure that the transactions are grouped by city.

[Seggellion]

I'm not sure if an interface is needed here?