Improve GUI course selection and authentication fallback

[DhrubaDC1]

Summary

• add an optional manual CAUTH input in the GUI when browser cookie loading fails
• load enrolled Coursera courses in the GUI on startup and via a refresh button
• allow selecting an enrolled course to automatically fill the course URL field while keeping manual URL entry available

Why

The existing GUI requires users to manually paste a course URL and depends entirely on browser cookie extraction for authentication. In environments where browser cookie loading is unreliable, this makes the GUI harder to use even when the user is already authenticated.
These changes improve the GUI workflow by:

• providing a manual authentication fallback
• showing enrolled courses directly in the app
• reducing the need to manually copy course URLs

Testing

• ran python3 -m py_compile maingui.py gui_components/help_text.py
• manually verified the GUI changes are wired for:
  • manual CAUTH entry
  • enrolled course loading on startup
  • manual refresh of enrolled courses
  • selecting a course autofills the course URL field
  • existing manual URL entry remains available

Notes

• data.bin is local runtime state and is not intended to be included in this PR.