EXTRA: Shop System

**ONLY ATTEMPT ONCE ALL OTHER MECHANICS ARE COMPLETE!**

- The player can, between waves, visit a shop.
- The shop's GUI should be activated once the player is within a certain, small distance to the shop, referred to as a "shop boundary".
- The shop should have its own GUI, where the player can buy perks to increase their walk speed, fire rate, health, or damage output.
- These perks should last the entirety of the player's current session.
- The player's total currency is decreased depending on the price of an item. (See issue "EXTRA: Currency System" for more information on currency)
- The player should be able to press the escape key to leave the shop.
- Upon leaving the shop, the player should not be able to reactivate the shop GUI until they first leave the "shop boundary" and re-enter it.
- The shop should not be able to be accessed by the player during waves.

開始前檢查現有的 wave 和 player 機制，並閱讀連結的 issue「EXTRA: Currency System」以了解對 currency 的依賴。定義 shop 的邊界、wave 之間的存取、貫穿整個 session 的 perk 行為、currency 消費、退出處理和重新進入規則；完成的標準是每個列出的互動都能正常運作，且無需在 wave 期間開啟 shop。

由索引模型根據 Issue 內容生成。