Add opt-in Xquik tweet backend

[kriptoburak]

Summary

• Add an opt-in Xquik backend for single-tweet posting with XCLI_BACKEND=xquik or X_BACKEND=xquik.
• Send supported posts to the public https://xquik.com/api/v1/x/tweets endpoint using XQUIK_API_KEY and XQUIK_ACCOUNT.
• Keep threads, replies, deletes, OAuth, and direct-token flows on the existing X API path.

Validation

• cargo fmt and cargo fmt check mode
• cargo clippy with warnings denied
• cargo test
• cargo build
• Added-line public-safety scan