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/StudyProgress.tsx

@@ -0,0 +1,57 @@
+'use client';
+
+import { Button } from '@/components/ui/button';
+import { Progress } from '@/components/ui/progress';
+import { BookOpen, X } from 'lucide-react';
+import { SKILLS_DEF, SPELLS_DEF } from '@/lib/game/constants';
+import { formatStudyTime } from '@/lib/game/formatting';
+import type { StudyTarget } from '@/lib/game/types';
+
+interface StudyProgressProps {
+  currentStudyTarget: StudyTarget | null;
+  skills: Record<string, number>;
+  studySpeedMult: number;
+  cancelStudy: () => void;
+}
+
+export function StudyProgress({
+  currentStudyTarget,
+  skills,
+  studySpeedMult,
+  cancelStudy,
+}: StudyProgressProps) {
+  if (!currentStudyTarget) return null;
+  
+  const target = currentStudyTarget;
+  const progressPct = Math.min(100, (target.progress / target.required) * 100);
+  const isSkill = target.type === 'skill';
+  const def = isSkill ? SKILLS_DEF[target.id] : SPELLS_DEF[target.id];
+  const currentLevel = isSkill ? (skills[target.id] || 0) : 0;
+  
+  return (
+    <div className="p-3 rounded border border-purple-600/50 bg-purple-900/20">
+      <div className="flex items-center justify-between mb-2">
+        <div className="flex items-center gap-2">
+          <BookOpen className="w-4 h-4 text-purple-400" />
+          <span className="text-sm font-semibold text-purple-300">
+            {def?.name}
+            {isSkill && ` Lv.${currentLevel + 1}`}
+          </span>
+        </div>
+        <Button
+          variant="ghost"
+          size="sm"
+          className="h-6 w-6 p-0 text-gray-400 hover:text-white"
+          onClick={cancelStudy}
+        >
+          <X className="w-4 h-4" />
+        </Button>
+      </div>
+      <Progress value={progressPct} className="h-2 bg-gray-800" />
+      <div className="flex justify-between text-xs text-gray-400 mt-1">
+        <span>{formatStudyTime(target.progress)} / {formatStudyTime(target.required)}</span>
+        <span>{studySpeedMult.toFixed(1)}x speed</span>
+      </div>
+    </div>
+  );
+}