fix(wechat-login): stop QR-code polling and setState after unmount (#2110)

* fix(wechat-login): stop QR-code polling and setState after unmount

checkQRCodeStatus rescheduled a new timer inside the in-flight .then
after the cleanup already ran, leaking a forever-poll and calling
setState on an unmounted component. Track a disposed flag and bail out
of both .then callbacks after unmount.

Fixes #2106

Co-Authored-By: Claude <noreply@anthropic.com>

* fix: isolate WeChat login polling generations

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Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: zgq <openai0229@gmail.com>
Co-authored-by: openai0229 <136558319+openai0229@users.noreply.github.com>