Created Adding a Course (Data Flow) (markdown)

Adding-a-Course-(Data-Flow).md

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+### Main functions
+- `addCourse` - Primary function for adding courses to schedules
+- `addCourseByURL` - Adding courses via direct URLs
+
+### User Interaction Points
+
+- `TableRow` - Adds action buttons to each course row
+- `CourseCatalogInjectedPopup` - Detailed course modal with add functionality
+- `HeadingAndActions` - Contains the actual "Add" button
+
+After `addCourse` is called, the course is appended to the active schedule's course array. Schedule's `updatedAt` timestamp is refreshed and schedules array is persisted to Chrome storage. Storage change triggers automatic synchronization across extension contexts through 
+```ts
+await UserScheduleStore.set('schedules', schedules);```
+