feat(#3): refactor and add lateral tree

[daniqss]

close #3

Pull Request

What does this PR introduce?

Clearly describe what this PR changes or adds.

• Refactor markdown parsing
• Add file tree lateral nav

Scope of change

Select the affected area(s):

• Builder (Markdown → HTML)
• Pseudocode Lexer
• Pseudocode Parser / AST
• Interpreter / Runtime
• Debugger (step execution)
• CLI
• Documentation
• Tests

Code Quality Checklist

Please confirm that you have followed the project's quality standards:

• I have run npm run lint, npm run format and fixed any errors.
• I have run ruff check --fix . and ruff format ..
• My commit messages follow the Conventional Commits format.

How has this been tested?

• Manual testing
• Unit tests added/updated
• Existing tests pass
• Verified generated HTML output
• Verified step-by-step debugger behavior

Explain briefly what was tested.

Breaking changes?

• No
• Yes (explain below)

If yes, describe:

• Posibly break old books

Additional notes

Anything reviewers should know?
Performance implications?
Design decisions?
Future follow-ups?