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/src/App.tsx

@@ -1,4 +1,4 @@
-import { lazy, Suspense, useEffect } from 'react';
+import { lazy, Suspense, useEffect, useState } from 'react';
 import Clarity from '@microsoft/clarity';
 import { Routes, Route, useLocation } from 'react-router-dom';
 import { Toaster } from 'sonner';
@@ -48,6 +48,19 @@ function LoadingFallback() {
   );
 }
 
+function IdleLoad({ children }: { children: React.ReactNode }) {
+  const [ready, setReady] = useState(false);
+  useEffect(() => {
+    if ('requestIdleCallback' in window) {
+      const id = requestIdleCallback(() => setReady(true));
+      return () => cancelIdleCallback(id);
+    }
+    const id = setTimeout(() => setReady(true), 2000);
+    return () => clearTimeout(id);
+  }, []);
+  return ready ? <>{children}</> : null;
+}
+
 export default function App() {
   useDirection();
   const location = useLocation();
@@ -152,7 +165,9 @@ export default function App() {
 
       <Footer />
       <Suspense fallback={null}>
-        <SiteAssistant />
+        <IdleLoad>
+          <SiteAssistant />
+        </IdleLoad>
         <CookieConsent />
       </Suspense>
       <Toaster