Fix skill study mana deduction in skillStore.ts

- Added useManaStore import to skillStore.ts
- Added mana deduction logic in startStudyingSkill action
- Mana is now properly deducted when starting to study a skill (unless already paid)

src/lib/game/stores/skillStore.ts

@@ -7,6 +7,7 @@ import { SKILLS_DEF, getStudySpeedMultiplier, getStudyCostMultiplier } from '../
 import type { StudyTarget, SkillUpgradeChoice } from '../types';
 import { SKILL_EVOLUTION_PATHS, getBaseSkillId } from '../skill-evolution';
 import { useCombatStore } from './combatStore';
+import { useManaStore } from './manaStore';
 
 export interface SkillState {
   // Skills
@@ -134,6 +135,10 @@ export const useSkillStore = create<SkillState>()(
         
         useCombatStore.getState().setAction('study');
         
+        if (!isAlreadyPaid && cost > 0) {
+          useManaStore.getState().spendRawMana(cost);
+        }
+        
         return { started: true, cost: isAlreadyPaid ? 0 : cost };
       },