Fix hair/vendor/loot bugs, revamp spellbook with tabs and icons, clean up action bar, add talent placeholder

- Fix white hair: always override M2 type-6 texture with DBC hair texture when available
- Fix vendor sell: add sellPrice to ItemDef/ItemTemplateRow, use directly instead of empty cache
- Fix empty loot: skip loot window when corpse has no items and no gold
- Revamp spellbook (P key): tabbed UI (General/Active/Passive), spell icons from SpellIcon.dbc, rank text
- Clean up action bar: only auto-populate Attack and Hearthstone, rest assigned via spellbook
- Add talent placeholder (N key): 3-tab window with level/talent point display
- Fix ffplay cleanup: non-blocking waitpid with SIGKILL fallback to prevent orphaned audio processes
- Fix pre-existing getQualityColor visibility for loot window rendering

include/ui/inventory_screen.hpp

@@ -126,6 +126,7 @@ private:
     game::EquipSlot getEquipSlotForType(uint8_t inventoryType, game::Inventory& inv);
     void renderHeldItem();
 
+public:
     static ImVec4 getQualityColor(game::ItemQuality quality);
 };