feat: implement item durability tracking and vendor repair

- Add ITEM_FIELD_DURABILITY (60) and ITEM_FIELD_MAXDURABILITY (61) to
  update_field_table.hpp enum and wotlk/update_fields.json
- Add curDurability/maxDurability to OnlineItemInfo and ItemDef structs
- Parse durability fields in OBJECT_CREATE and OBJECT_VALUES handlers;
  preserve existing values on partial updates (fixes stale durability
  being reset to 0 on stack-count-only updates)
- Propagate durability to ItemDef in all 5 rebuildOnlineInventory() paths
- Implement GameHandler::repairItem() and repairAll() via CMSG_REPAIR_ITEM
  (itemGuid=0 repairs all equipped items per WotLK protocol)
- Add canRepair flag to ListInventoryData; set it when player selects
  GOSSIP_OPTION_ARMORER in gossip window
- Show "Repair All" button in vendor window header when canRepair=true
- Display color-coded durability in item tooltip (green >50%, yellow
  >25%, red <=25%)

Data/expansions/wotlk/update_fields.json

@@ -33,6 +33,8 @@
   "PLAYER_EXPLORED_ZONES_START": 1041,
   "GAMEOBJECT_DISPLAYID": 8,
   "ITEM_FIELD_STACK_COUNT": 14,
+  "ITEM_FIELD_DURABILITY": 60,
+  "ITEM_FIELD_MAXDURABILITY": 61,
   "CONTAINER_FIELD_NUM_SLOTS": 64,
   "CONTAINER_FIELD_SLOT_1": 66
 }