chore: backend fixes + vite config + AppShell cleanup

- pyproject.toml: add httpx>=0.27.0 dependency.
- frontend/vite.config.js: bind port to 31400, strictPort, proxy /api -> 31401.
- start.sh: rewrite as detached daemon (nohup + disown + PID files);
  re-running is treated as restart.
- .gitignore: ignore .run/ (PID files).
- frontend AppShell.vue: drop unused GitHub Star link.

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

.gitignore

@@ -28,6 +28,7 @@ frontend/node_modules/.vite/
 # 本地启动/部署日志
 backend.log
 frontend.log
+.run/
 
 # 临时文件
 *.swp