Add route-based code splitting and loading skeletons

- Lazy-load Dashboard, Comparison, Slides via React.lazy + Suspense
- Main bundle reduced from 606KB to 256KB
- Replace full-screen spinner with skeleton cards during load
- Skeleton used for both initial data fetch and route transitions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/shared/LoadingSkeleton.tsx

@@ -0,0 +1,27 @@
+import React from 'react';
+
+function SkeletonCard({ wide = false }: { wide?: boolean }) {
+  return (
+    <div className={`skeleton-card ${wide ? 'skeleton-card-wide' : ''}`}>
+      <div className="skeleton-line skeleton-line-short" />
+      <div className="skeleton-line skeleton-line-tall" />
+    </div>
+  );
+}
+
+export default function LoadingSkeleton() {
+  return (
+    <div className="skeleton-container">
+      <div className="skeleton-stats">
+        <SkeletonCard />
+        <SkeletonCard />
+        <SkeletonCard />
+        <SkeletonCard />
+      </div>
+      <div className="skeleton-charts">
+        <SkeletonCard wide />
+        <SkeletonCard wide />
+      </div>
+    </div>
+  );
+}