Fix issue #22 course visibility and replacement drop

[ceilf6]

Summary

• document that target courses must be loaded into the current page DOM and recommend precise course-code search
• add usage notes for "点此查看更多", course detail expansion, refresh behavior, and replacement-course risk
• fix replacement-course drop lookup by reading the selected-course right list before falling back to the old teaching-class lookup
• add a Node regression test for locating the drop button in the selected-course right list

Verification

• node --check courseGrabber.js
• node --test
• git diff --check HEAD~1 HEAD

Fixes #22

[ceilf6]

Closing this develop-targeted PR because repo-guard only runs for PRs targeting master. Recreated the same issue #22 fix as #24 against master.