fix: ECH for mihomo and sing-box

[mdjvd]

Summary

FYI: This is AI-written code
We've had ECH for Xray, but not for mihomo, and the implementation for Sing-Box was imperfect and unusable.
This PR fixes the ECH issue for both subscription formats.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor / cleanup
• Documentation
• Tests / CI

Checklist

• I tested the change locally or explained why it cannot be tested.
• I added or updated tests for behavior changes.
• I updated documentation, translations, or examples if needed.
• I checked database migrations when models or schema changed.
• I did not include secrets, tokens, private keys, or unrelated changes.

Testing

I have tested it on both formats, and they work with no issues as far as I can see.

Screenshots

Notes for reviewers

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[mdjvd]

If the ech is a base-64 encoded config, there is no problem. But if we choose to use the other supported format by Xray (eg, "example.com+https://1.1.1.1/dns-query"), then Sing-box and Mihomo will not be able to start the profile and proxies at all.
So I think we have to have different fields for each format in hosts.