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/vite.config.ts

@@ -88,6 +88,14 @@ export default defineConfig({
             return 'editor';
           }
 
+          if (id.includes('lucide-react')) {
+            return 'icons';
+          }
+
+          if (id.includes('@microsoft/clarity')) {
+            return 'analytics';
+          }
+
           return undefined;
         },
       },