feat: add standalone interactive practice UI

[HareshKarnan]

This PR introduces a standalone, modern web interface for practicing PyTorch implementations, providing a more focused and platform-native experience compared to the traditional Jupyter Notebook workflow.

Key Features:

• Dual-Pane Layout: Integrated problem description on the left and a professional-grade Monaco code editor on the right.
• Real-Time Feedback: Execute code and receive instant, structured test results with pass/fail indicators, execution time, and detailed error tracebacks.
• LaTeX Math Rendering: Problem descriptions and hints now render mathematical formulas beautifully using KaTeX.
• Auto-Persistence: User code is automatically saved to browser local storage, preventing data loss on reloads.
• Progress Tracking: Automatically marks problems as solved when all tests pass, with visual indicators in the problem viewer and a difficulty-grouped task selector.
• Pure English: All original Chinese comments and instructions have been translated to English for broader accessibility.

Technical Architecture:

• Frontend: Next.js (React) with Tailwind CSS and Monaco Editor.
• Backend: FastAPI (Python) serving a custom execution engine (web_engine.py) that safely executes user code strings in isolated namespaces.
• Integration: Automatically parses existing Jupyter templates to extract problem content and initial code snippets.

How to run:

1. pip install -r api/requirements.txt && python -m uvicorn api.main:app --port 8000 --reload
2. cd web && npm install && npm run dev
3. Visit http://localhost:3000