perf: Optimize PageSpeed performance score

- Replace 2.2MB social-preview.svg with lightweight 1KB vector SVG
- Remove lucide-react manual chunk (rely on tree-shaking instead)
- Separate react-router-dom into its own 'router' chunk
- Add build.target es2020 and chunkSizeWarningLimit
- Add modulepreload hint for main entry point
- Google Fonts already uses media=print non-blocking pattern
- i18n already lazy-loads ar/fr dynamically
- AdSlot already loads AdSense dynamically with intersection observer

Expected improvement: 34 → 70+ on mobile PageSpeed

frontend/index.html

@@ -65,6 +65,7 @@
   <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>