refactor: Major codebase refactoring for maintainability

Store refactoring (2138 → 1651 lines, 23% reduction):
- Extract computed-stats.ts with 18 utility functions
- Extract navigation-slice.ts for floor navigation actions
- Extract study-slice.ts for study-related actions
- Move fmt/fmtDec to computed-stats, re-export from formatting

Page refactoring (2554 → 1695 lines, 34% reduction):
- Use existing SpireTab component instead of inline render
- Extract ActionButtons component
- Extract CalendarDisplay component
- Extract CraftingProgress component
- Extract StudyProgress component
- Extract ManaDisplay component
- Extract TimeDisplay component
- Create tabs/index.ts for cleaner exports

This improves code organization and makes the codebase more maintainable.

src/lib/game/formatting.ts

@@ -4,6 +4,9 @@
 import { ELEMENTS } from '@/lib/game/constants';
 import type { SpellCost } from '@/lib/game/types';
 
+// Re-export number formatting functions from computed-stats.ts
+export { fmt, fmtDec } from './computed-stats';
+
 /**
  * Format a spell cost for display
  */