perf: optimize frontend bundle - reduce main chunk 77%

- vite.config: separate lucide-react icons + analytics into own chunks
- App.tsx: defer SiteAssistant loading via requestIdleCallback
- HeroUploadZone: lazy-load ToolSelectorModal + dynamic import fileRouting
- HeroUploadZone: add aria-label on dropzone input (accessibility)
- SocialProofStrip: defer API call until component is in viewport
- index.html: remove dev-only modulepreload hint

Main bundle: 266KB -> 61KB (-77%)

frontend/index.html

@@ -65,7 +65,6 @@
   <noscript>
     <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Inter:wght@400;600;700&family=Tajawal:wght@400;700&display=swap" />
   </noscript>
-  <link rel="modulepreload" href="/src/main.tsx" />
   <title>Dociva — Free Online File Tools</title>
 </head>