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.tsx

@@ -1,8 +1,10 @@
-import React, { useState, useEffect, useCallback, ReactNode } from 'react';
+import React, { useState, useEffect, useCallback, ReactNode, lazy, Suspense } from 'react';
 import { BrowserRouter as Router, Routes, Route, Link, useLocation } from 'react-router-dom';
-import Dashboard from './components/Dashboard';
-import Comparison from './components/Comparison';
-import Slides from './components/Slides';
+
+const Dashboard = lazy(() => import('./components/Dashboard'));
+const Comparison = lazy(() => import('./components/Comparison'));
+const Slides = lazy(() => import('./components/Slides'));
+import LoadingSkeleton from './components/shared/LoadingSkeleton';
 import { fetchData, getCacheStatus, refreshData } from './services/dataService';
 import { useLanguage } from './contexts/LanguageContext';
 import type { MuseumRecord, CacheStatus, DataErrorType } from './types';
@@ -83,9 +85,8 @@ function App() {
 
   if (loading) {
     return (
-      <div className="loading-container" dir={dir}>
-        <div className="loading-spinner"></div>
-        <p>{t('app.loading')}</p>
+      <div className="app" dir={dir}>
+        <LoadingSkeleton />
       </div>
     );
   }
@@ -191,11 +192,13 @@ function App() {
           </div>
         </nav>
         
-        <Routes>
-          <Route path="/" element={<Dashboard data={data} showDataLabels={showDataLabels} setShowDataLabels={setShowDataLabels} includeVAT={includeVAT} setIncludeVAT={setIncludeVAT} />} />
-          <Route path="/comparison" element={<Comparison data={data} showDataLabels={showDataLabels} setShowDataLabels={setShowDataLabels} includeVAT={includeVAT} setIncludeVAT={setIncludeVAT} />} />
-          <Route path="/slides" element={<Slides data={data} />} />
-        </Routes>
+        <Suspense fallback={<LoadingSkeleton />}>
+          <Routes>
+            <Route path="/" element={<Dashboard data={data} showDataLabels={showDataLabels} setShowDataLabels={setShowDataLabels} includeVAT={includeVAT} setIncludeVAT={setIncludeVAT} />} />
+            <Route path="/comparison" element={<Comparison data={data} showDataLabels={showDataLabels} setShowDataLabels={setShowDataLabels} includeVAT={includeVAT} setIncludeVAT={setIncludeVAT} />} />
+            <Route path="/slides" element={<Slides data={data} />} />
+          </Routes>
+        </Suspense>
         
         {/* Mobile Bottom Navigation */}
         <nav className="mobile-nav">