Fish market: sell fish for currency

[stphung]

Intent

A market to sell caught fish for currency — an economy source/sink for the fishing system.

Open questions

• Which currency — gold (see Gold system and item upgrades #28), PR, or a fishing-specific currency?
• Flat prices per species/rank, or dynamic/rotating prices (Stormglass-style daily rotation)?
• Where does it live in the UI — the fishing overlay, or its own screen?

Status: direction chosen, mechanics undesigned. Pairs with #28 (gold system) and #228 (dungeon fishing).