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/components/game/tabs/index.ts

@@ -0,0 +1,7 @@
+// ─── Tab Components Index ──────────────────────────────────────────────────────
+// Re-exports all tab components for cleaner imports
+
+export { CraftingTab } from './CraftingTab';
+export { SpireTab } from './SpireTab';
+export { SpellsTab } from './SpellsTab';
+export { LabTab } from './LabTab';