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/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-06-08T13:03:24.191Z",
+    "generated": "2026-06-08T13:51:43.381Z",
     "description": "Import dependency graph for src/lib/game. Keys are files, values are arrays of files they import.",
     "usage": "To find what a file affects, search for its path in the VALUES. To find what a file depends on, look at its KEY entry."
   },