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

src/ui/game_screen.cpp

@@ -103,6 +103,9 @@ void GameScreen::render(game::GameHandler& gameHandler) {
     // Spellbook (P key toggle handled inside)
     spellbookScreen.render(gameHandler, core::Application::getInstance().getAssetManager());
 
+    // Talents (N key toggle handled inside)
+    talentScreen.render(gameHandler);
+
     // Set up inventory screen asset manager + player appearance (once)
     {
         static bool inventoryScreenInit = false;