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

CMakeLists.txt

@@ -154,6 +154,7 @@ set(WOWEE_SOURCES
     src/ui/inventory_screen.cpp
     src/ui/quest_log_screen.cpp
     src/ui/spellbook_screen.cpp
+    src/ui/talent_screen.cpp
 
     # Main
     src/main.cpp
@@ -241,6 +242,7 @@ set(WOWEE_HEADERS
     include/ui/game_screen.hpp
     include/ui/inventory_screen.hpp
     include/ui/spellbook_screen.hpp
+    include/ui/talent_screen.hpp
 )
 
 # Create executable