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/App.css

@@ -1987,3 +1987,74 @@ html[dir="rtl"] .chart-export-btn.visible {
   direction: ltr !important;
   text-align: left !important;
 }
+
+/* ========================================
+   Loading Skeleton
+   ======================================== */
+
+@keyframes skeleton-pulse {
+  0%, 100% { opacity: 0.4; }
+  50% { opacity: 1; }
+}
+
+.skeleton-container {
+  padding: 80px 24px 24px;
+  max-width: 1200px;
+  margin: 0 auto;
+}
+
+.skeleton-stats {
+  display: grid;
+  grid-template-columns: repeat(4, 1fr);
+  gap: 16px;
+  margin-bottom: 24px;
+}
+
+.skeleton-card {
+  background: #f1f5f9;
+  border-radius: 12px;
+  padding: 20px;
+  animation: skeleton-pulse 1.5s ease-in-out infinite;
+}
+
+.skeleton-card-wide {
+  grid-column: span 2;
+}
+
+.skeleton-line {
+  background: #e2e8f0;
+  border-radius: 6px;
+}
+
+.skeleton-line-short {
+  width: 40%;
+  height: 14px;
+  margin-bottom: 12px;
+}
+
+.skeleton-line-tall {
+  width: 100%;
+  height: 120px;
+}
+
+.skeleton-card-wide .skeleton-line-tall {
+  height: 200px;
+}
+
+.skeleton-charts {
+  display: grid;
+  grid-template-columns: repeat(2, 1fr);
+  gap: 16px;
+}
+
+@media (max-width: 768px) {
+  .skeleton-stats {
+    grid-template-columns: repeat(2, 1fr);
+  }
+  .skeleton-charts {
+    grid-template-columns: 1fr;
+  }
+  .skeleton-card-wide {
+    grid-column: span 1;
+  }
+}