refactor(core): flatten DTO hierarchy to align with flat backend API

[syed-tp]

• The nested DTO structure created an architectural "trap" by implying hierarchical data where none exists at the source. Since the backend API and local database are strictly flat, keeping nested collections (chapters/lessons) led to inconsistent state and potential null pointer bugs if the nesting wasn't manually populated.
• This refactor enforces the Single Source of Truth by requiring independent, ID-based fetching through repository streams, which results in more predictable data flow across the UI.

[gemini-code-assist]

Code Review

This pull request implements DTO hierarchy flattening by removing nested collections from CourseDto and ChapterDto, aligning the models with the flat backend API and local database. The changes include updating serialization logic, refactoring Riverpod providers to fetch related data independently, and adjusting UI screens to handle separate data streams. Feedback identifies an N+1 network request bottleneck in lesson fetching, non-reactive provider implementations using one-time stream reads, and potential UI flickering during loading states.

[chatgpt-codex-connector]

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