Add opt-in Xquik tweet backend

[kriptoburak]

Summary

• Add an opt-in Xquik backend selected with X_BACKEND=xquik or TWITTER_BACKEND=xquik.
• Send text posts to the public https://xquik.com/api/v1/x/tweets endpoint with XQUIK_API_KEY and XQUIK_ACCOUNT.
• Keep the existing hosted backend as the default path.

Validation

• cd src/cli && go test ./...
• cd src/cli && go build ./...
• Diff whitespace check
• Public-safety scan of added lines

[vercel]

@kriptoburak is attempting to deploy a commit to the devhindo's projects Team on Vercel.

A member of the Team first needs to authorize it.

[devhindo]

hello @kriptoburak! I just sent you an email, would appreciate if you check it ..