Fix 3 files: migrate to useSkillStore, remove parallel study UI

- SkillsTab.tsx: Replace useGameStore with useSkillStore for currentStudyTarget
- SkillUpgradeDialog.tsx: Migrate to useSkillStore, fix commitSkillUpgrades call
- SkillRow.tsx: Remove parallel study feature (startParallelStudySkill doesn't exist in skillStore), remove hasParallelStudy reference
- Update index.ts export to point to new SkillsTab location
- Delete old legacy tabs/SkillsTab.tsx file

docs/project-structure.txt

@@ -129,7 +129,6 @@ Mana-Loop/
 │   │   │   │   ├── SkillCategoryHeader.tsx
 │   │   │   │   ├── SkillMultipliers.tsx
 │   │   │   │   ├── SkillRow.tsx
-│   │   │   │   ├── SkillsTab.tsx
 │   │   │   │   ├── SpellsTab.tsx
 │   │   │   │   ├── SpireHeader.tsx
 │   │   │   │   ├── SpireTab.tsx