Update: Move themeVariables storage from course to config

[taylortom]

Update

• Read themeVariables from config record first, falling back to course for unmigrated data
• Apply/remove preset handlers now target the config record instead of the course record
• processFileVariables and processCustomStyling accept variables parameter directly
• Version-based LESS generation: skip LESS variable string when framework >= 6.61.0 handles it natively
• Register import content migration to move course.themeVariables → config.themeVariables on course import
• Add semver dependency

Testing

1. Start with npm run dev from adapt-authoring/
2. Open theme editor, apply a preset — verify MongoDB db.content.findOne({ _type: 'config', _courseId: ... }) has themeVariables
3. Preview/publish a course — verify config.json in build output contains themeVariables
4. Check built CSS has expected variable overrides
5. Test with unmigrated course data (themeVariables on course record) — build should still work via fallback
6. Import a course with old-format themeVariables on course — verify migration moves it to config

Fixes #55
Depends on adapt-security/adapt-authoring-ui#449
Related framework PR: adaptlearning/adapt_framework#3578