feat: add "Usable" filter to auction house and query token item names

Add a "Usable" checkbox to the AH search UI that filters results to
items the player can actually equip/use (server-side filtering via the
usableOnly parameter in CMSG_AUCTION_LIST_ITEMS). Also ensure token
item names for extended costs are queried from the server via
ensureItemInfo() so they display properly instead of "Item#12345".

include/ui/game_screen.hpp

@@ -581,6 +581,7 @@ private:
     uint32_t auctionBrowseOffset_ = 0;       // Pagination offset for browse results
     int auctionItemClass_ = -1;              // Item class filter (-1 = All)
     int auctionItemSubClass_ = -1;           // Item subclass filter (-1 = All)
+    bool auctionUsableOnly_ = false;         // Filter to items usable by current class/level
 
     // Guild bank money input
     int guildBankMoneyInput_[3] = {0, 0, 0};  // gold, silver, copper