Fix spellbook tabs race condition with DBC lazy loading

lookupSpellName() (called by action bar tooltips) was triggering
loadSpellDBC() early, setting dbcLoadAttempted=true before the
spellbook opened. This caused loadSkillLineDBCs() to be skipped,
so all spells were categorized into "General". Fixed by checking
each DBC's own loaded flag independently and re-categorizing when
skill line data becomes available.

include/ui/spellbook_screen.hpp

@@ -71,6 +71,7 @@ private:
     // Categorized spell tabs
     std::vector<SpellTabInfo> spellTabs;
     size_t lastKnownSpellCount = 0;
+    bool categorizedWithSkillLines = false;
 
     // Search filter
     char searchFilter_[128] = "";