fix: static lastSpellCount shared across SpellHandler instances

The spellbook tab dirty check used a function-local static, meaning
switching to a character with the same spell count would skip the
rebuild and return the previous character's tabs. Changed to an
instance member so each SpellHandler tracks its own count.

include/game/spell_handler.hpp

@@ -307,6 +307,7 @@ private:
 
     // Spell book tabs
     std::vector<SpellBookTab> spellBookTabs_;
+    size_t lastSpellCount_ = 0;
     bool spellBookTabsDirty_ = true;
 
     // Talent wipe confirm dialog