feat: Tailwind offcanvas UI with split code/result workspace

[edumgt]

Motivation

• Provide a more modern, compact UI where the chapter list is an offcanvas menu and the main area shows code and result side-by-side for easier comparison.
• Allow the frontend to display the raw practice.py source so users can view code before or after running it.

Description

• Reworked the frontend into a Tailwind-based layout with a left offcanvas chapter menu and a sticky header in frontend/index.html.
• Changed the main workspace to a 2-column split view (left: Python source #source, right: execution result #result) and added UI controls to open/close the menu.
• Implemented interactive chapter list behavior in frontend/app.js so clicking a chapter loads the source and the 실행 button loads source and runs the chapter, closing the offcanvas after actions.
• Added backend API GET /api/chapters/{chapter_id}/source by updating backend/app/main.py to return the practice.py content as a ChapterSourceResponse.

Testing

• Compiled Python files with python -m compileall to verify syntax and the compilation succeeded.
• Started the FastAPI app with uvicorn to validate the server started and gracefully shut down after verification.
• Queried the API with curl and confirmed both /api/chapters and GET /api/chapters/chapter01/source returned expected responses.
• Ran an automated Playwright script that opened the UI, triggered the offcanvas menu and ran a chapter, and successfully produced a screenshot of the offcanvas + split view.

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Codex Task