Fix SPA routing for GitHub Pages deployment

- Update 404.html with React content instead of Vue
- Add automatic 404.html copy to build script
- Fix URL typo in og:url meta tag
- Ensure client-side routing works for user pages

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "scripts": {
     "dev": "vite",
-    "build": "tsc && vite build",
+    "build": "tsc && vite build && cp dist/index.html dist/404.html",
     "preview": "vite preview",
     "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0"
   },