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/index.ts

@@ -5,7 +5,6 @@
 export { CraftingTab } from './tabs/CraftingTab';
 export { SpireTab } from './tabs/SpireTab';
 export { SpellsTab } from './tabs/SpellsTab';
-export { LabTab } from './tabs/LabTab';
 export { SkillsTab } from './SkillsTab';
 export { StatsTab } from './tabs/StatsTab';