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>

.claude/settings.local.json

@@ -19,7 +19,8 @@
       "Bash(find:*)",
       "Bash(rm:*)",
       "Bash(grep:*)",
-      "Bash(ls:*)"
+      "Bash(ls:*)",
+      "Bash(cp:*)"
     ],
     "deny": []
   }