Centralization of Equipment Slot specifications

[Volte6]

Description

Centralizes the equipment slot system so that the canonical list of slots lives in one place (items.AllEquipSlots()), eliminating large per-slot switch and if/else blocks that had to be updated everywhere a new slot was added or removed.

Changes

• Added AllEquipSlots(), WeaponSlots(), and ArmorSlots() to internal/items as the single source of truth for the equipment slot list
• Added Get(slot) and Set(slot, item) accessors to Worn so all slot access goes through one switch rather than being scattered across the codebase
• Added SlotLabel(slot) to Worn so UI label strings are defined in one place instead of hard-coded in rendering code
• Replaced all per-slot switch/if-else chains in character.go (defense, validation, equip, unequip, uncurse, gear value, etc.) with loops over AllSlots()
• Updated combat, mobs, races, inventory/status panels, and the GMCP module to use the new slot iterators instead of their own local slot lists
• Updated admin HTML templates and the gear window JS to derive slot lists dynamically rather than hard-coding them
• Added a new items API endpoint to expose the slot list to the web layer
• Added worn_test.go with tests covering Get/Set/SlotLabel and the slot helper functions