[codex] fix dashboard and tun proxy controls

[jawwe]

Changes

• Fix zashboard UI upgrade by generating a real external-ui-url for the zashboard dist.zip asset instead of /ui.
• Add combined Tun/system-proxy commands:
  • clashctl tun on-proxy-off / clashctl tun proxy-off: enable Tun, then disable system proxy.
  • clashctl tun off-proxy-on / clashctl tun proxy-on: disable Tun, then enable system proxy.
• Extend clashctl doctor to detect when Tun and system proxy are both enabled, including problem, evidence, recommendation, and detailed check output.
• Wire clashctl tun logs to mihomo logs.
• Update README for zashboard external-ui-url, Tun combined commands, and the doctor check.

Validation

• git diff --check
• bash -n install.sh uninstall.sh scripts/core/*.sh scripts/init/*.sh .github/scripts/*.sh scripts/dev/*.sh
• GitHub Actions Text Encoding passed.

Notes

• Per request, workflow/check script changes are not included in this PR. The PR only contains product code and README updates.