feat: remove Spells tab UI

- Remove Spells tab trigger from TabTriggers in page.tsx
- Remove TabsContent value='spells' block in page.tsx
- Remove lazy import of SpellsTab in page.tsx
- Change default activeTab from 'spells' to 'disciplines'
- Remove SpellsTab re-export from tabs/index.ts
- Remove SpellsTab re-export from game/index.ts
- Delete src/components/game/tabs/SpellsTab.tsx

Spell data (SPELLS_DEF, spells store state) preserved - spells still exist as enchantments.

docs/project-structure.txt

@@ -145,7 +145,6 @@ Mana-Loop/
 │   │   │   │   ├── GuardianPactsTab.tsx
 │   │   │   │   ├── PrestigeTab.test.ts
 │   │   │   │   ├── PrestigeTab.tsx
-│   │   │   │   ├── SpellsTab.tsx
 │   │   │   │   ├── SpireSummaryTab.helpers.tsx
 │   │   │   │   ├── SpireSummaryTab.test.ts
 │   │   │   │   ├── SpireSummaryTab.tsx