feat(updater): link stable and alpha repositories

[ShiinaKuroko]

Summary

• add an explicit Stable/Alpha update-channel selector
• use yltx/AutoWSGR-GUI + OpenWSGR/AutoWSGR for Stable
• use ShiinaKuroko/AutoWSGR-GUI + ShiinaKuroko/AutoWSGR for Alpha
• invalidate the managed backend marker after channel changes so the backend switches on restart
• keep downgrade disabled after electron-updater changes channels
• warn that preview builds may contain unfixed bugs while exposing in-development features

Review

• GUI repository, update channel, accepted candidates, and backend distribution now derive from the same persisted setting
• Stable and Alpha backend sources are pinned to immutable commits
• external backend mode remains unaffected

Verification

• npm run test:build
• node scripts/tests/test-main-services.js
• node scripts/tests/test-main-ipc.js
• node scripts/tests/test-python-environment.js
• node scripts/tests/test-backend-distribution.js
• npm run test:settings
• git diff --check

This carries the reviewed channel integration on top of the existing GUI 2.1 bridge work from #29 because its upstream-owned head branch is not writable from the contributor account.

[ShiinaKuroko]

Superseded by #31, which uses the required ShiinaKuroko:ShiinaKuroko source branch.