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/CalendarDisplay.tsx

@@ -0,0 +1,44 @@
+'use client';
+
+import { Tooltip, TooltipContent, TooltipTrigger } from '@/components/ui/tooltip';
+import { MAX_DAY, INCURSION_START_DAY } from '@/lib/game/constants';
+
+interface CalendarDisplayProps {
+  currentDay: number;
+}
+
+export function CalendarDisplay({ currentDay }: CalendarDisplayProps) {
+  const days: React.ReactElement[] = [];
+  
+  for (let d = 1; d <= MAX_DAY; d++) {
+    let dayClass = 'w-7 h-7 rounded text-xs flex items-center justify-center font-mono border transition-all ';
+    
+    if (d < currentDay) {
+      dayClass += 'bg-blue-900/30 border-blue-800/50 text-blue-400';
+    } else if (d === currentDay) {
+      dayClass += 'bg-blue-600/40 border-blue-500 text-blue-300 shadow-lg shadow-blue-500/30';
+    } else {
+      dayClass += 'bg-gray-800/30 border-gray-700/50 text-gray-500';
+    }
+    
+    if (d >= INCURSION_START_DAY) {
+      dayClass += ' border-red-600/50';
+    }
+    
+    days.push(
+      <Tooltip key={d}>
+        <TooltipTrigger asChild>
+          <div className={dayClass}>
+            {d}
+          </div>
+        </TooltipTrigger>
+        <TooltipContent>
+          <p>Day {d}</p>
+          {d >= INCURSION_START_DAY && <p className="text-red-400">Incursion Active</p>}
+        </TooltipContent>
+      </Tooltip>
+    );
+  }
+  
+  return <>{days}</>;
+}