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/components/game/tabs/index.ts

@@ -4,7 +4,6 @@
 export { CraftingTab } from './CraftingTab';
 export { SpireTab } from './SpireTab';
 export { SpellsTab } from './SpellsTab';
-export { LabTab } from './LabTab';
 // SkillsTab is now exported from src/components/game/index.ts
 export { SkillsTab } from '../SkillsTab';
 export { StatsTab } from './StatsTab';