feat: add auto-sell grey items on vendor open

When 'Auto Sell Greys' is enabled in Settings > Gameplay, all grey
(ItemQuality::POOR) items in the backpack and extra bags are sold
automatically when opening a vendor window. Items with no sell price
are skipped. A system chat message reports the number of items sold
and total gold received. The setting persists to ~/.wowee/settings.cfg
under the key auto_sell_grey.

include/ui/game_screen.hpp

@@ -195,6 +195,7 @@ private:
     bool pendingSeparateBags = true;
     bool pendingShowKeyring = true;
     bool pendingAutoLoot = false;
+    bool pendingAutoSellGrey = false;
 
     // Keybinding customization
     int pendingRebindAction = -1;  // -1 = not rebinding, otherwise action index