fix: use authoritative autocast state for pet action bar and correct tooltip labels

- Use isPetSpellAutocast() instead of parsing the slot value high byte for
  autocast detection; the authoritative source is the SMSG_PET_SPELLS spell
  list activeFlags, not the action bar slot value.
- Fix tooltip mapping: actionId==2 maps to "Follow", actionId==5 to "Attack",
  others to "Stay" (removed erroneous duplicate Follow case for actionId==4).
- Update spellbook comment: TBC Spell.dbc has ~220+ fields (not ~167).

src/ui/spellbook_screen.cpp

@@ -45,7 +45,7 @@ void SpellbookScreen::loadSpellDBC(pipeline::AssetManager* assetManager) {
     }
 
     uint32_t fieldCount = dbc->getFieldCount();
-    // Classic 1.12 Spell.dbc has 148 fields (Tooltip at index 147), TBC has ~167, WotLK has 234.
+    // Classic 1.12 Spell.dbc has 148 fields (Tooltip at index 147), TBC has ~220+ (SchoolMask at 215), WotLK has 234.
     // Require at least 148 fields so all expansions can load spell names/icons via the DBC layout.
     if (fieldCount < 148) {
         LOG_WARNING("Spellbook: Spell.dbc has ", fieldCount, " fields, too few to load");