feat: add Sort Bags button to backpack window

Adds Inventory::sortBags() which collects all items from the backpack
and equip bags, sorts them client-side by quality descending → item ID
ascending → stack count descending, then writes them back. A "Sort Bags"
SmallButton is rendered in the backpack footer with a tooltip explaining
the sort order.

The sort is purely local (no server packets) since the WoW protocol has
no sort-bags opcode; it provides an instant, session-persistent visual
reorder.

include/game/inventory.hpp

@@ -125,6 +125,10 @@ public:
     int findFreeBackpackSlot() const;
     bool addItem(const ItemDef& item);
 
+    // Sort all bag slots (backpack + equip bags) by quality desc → itemId asc → stackCount desc.
+    // Purely client-side: reorders the local inventory struct without server interaction.
+    void sortBags();
+
     // Test data
     void populateTestItems();