Keep comments sidebar applied after refresh

[puneetdixit200]

Fixes #3880

Summary

• reapply the comments sidebar layout when YouTube rebuilds the watch page DOM
• guard the sidebar setup while comments/related/sidebar nodes are still loading
• avoid stacking duplicate scrollbar styles and resize observers
• add a regression test for refresh/default-layout restoration and late-loading comments

Tests

• npm_config_cache=$PWD/.npm-cache npx jest tests/unit/comments-sidebar.test.js --runInBand
• npm_config_cache=$PWD/.npm-cache npm test -- --runInBand
• npm_config_cache=$PWD/.npm-cache npx jest --runInBand
• npm_config_cache=$PWD/.npm-cache npm run lint
• git diff --check