feat: add leetcode.cn support to TUI login flow

[dong-frank]

Summary

I first tested the current leetcode.cn flow directly in the built CLI with:

node dist/index.js login   # pick leetcode.cn
node dist/index.js daily
node dist/index.js stats

Those worked correctly on my side.

After that, I tried logging in through the TUI and found that the TUI login flow still did not support selecting leetcode.cn directly. This PR adds that missing TUI implementation.

What This PR Changes

• add a site-selection step to the TUI login flow
• allow choosing leetcode.com or leetcode.cn directly inside TUI login
• update the login instructions to show the correct cookie domain for the selected site
• persist the selected site before validating credentials
• configure the site-aware LeetCode client before auth checks
• add tests covering the new TUI login flow