Add disabled_section_types to disable LLM section classification

[nicpottier]

Summary

Adds disabled_section_types config array to exclude section types from LLM classification without destructively deleting them. Users can now disable a type via the X button in the Storyboard Settings and easily re-enable it later. The type definition, description, and render strategy mappings are preserved.

Key changes:

• Pipeline filters disabled types before sending to LLM, so they never appear in classification options
• UI replaces destructive "delete" with non-destructive "disable/enable" toggle (X button changes color when disabled)
• Activity master toggle now reflects "any activities enabled" state instead of "all enabled"
• Added test coverage for disabled type filtering

Implementation details

• Schema: Added disabled_section_types: z.array(z.string()).optional() to AppConfig
• Pipeline: buildSectioningConfig filters disabled types from sectionTypes array
• UI: Section types table rows can be disabled (dimmed, strikethrough). Disabled types are excluded from dropdowns in Storyboard and Quizzes settings
• Activity toggle: Simplified to only manage disabled_section_types — render strategy mappings are now left untouched when disabling