feat: Add AI Doubt Solver and 30-Day Python Challenge Tracker

[Hemasri0405]

Summary

This PR introduces two new learning features to the PyBe platform:

• AI-powered Doubt Solver — Paste Python code and get beginner-friendly error explanations
• 30-Day Python Challenge — Progressive coding challenges with progress tracking and badges

Features Added

AI Doubt Solver (/doubts)

• Paste Python code into a large text area
• Optional question field for specific concerns
• Explain My Error button triggers analysis
• Detects syntax errors (mismatched brackets, missing colons, unclosed strings) and logical errors (infinite loops, undefined variables, assignment vs comparison)
• Card layout with sections: Error Found, Simple Explanation, Hint, Corrected Code, Learn More
• Full doubt history with reload and delete
• Error handling for empty input and API failures

30-Day Python Challenge (/challenge)

• 30 challenges (Days 1–30: Easy → Medium → Hard)
• Day selector grid with difficulty color coding
• Problem statement, example input/output, hint button per challenge
• Solution text area and Mark as Completed button
• Progress tracking: completed days, daily streak, percentage
• Badge system: Beginner (1), Consistent Learner (7), Python Explorer (15), Challenge Master (30)
• Progress persisted in backend db.json

Backend Changes

New Routes

Method	Endpoint	Description
POST	/api/doubts	Analyze Python code
GET	/api/doubts	List doubt history
DELETE	/api/doubts/:id	Delete a doubt entry
GET	/api/challenges	List all 30 challenges
GET	/api/challenges/progress	Get completion progress
POST	/api/challenges/:day/complete	Mark a day as completed

New Files

• server/src/services/doubtSolver.js — Rule-based Python code analyzer
• server/src/data/challenges.js — 30 challenge definitions
• server/src/routes/doubts.js — Doubt solver API routes
• server/src/routes/challenges.js — Challenge tracker API routes

Updated Files

• server/src/index.js — Registered new routes
• server/src/data/store.js — Added doubt/challenge CRUD operations

Frontend Changes

New Files

• client/src/pages/DoubtSolver.jsx — Doubt Solver page
• client/src/pages/PythonChallenge.jsx — Challenge page

Updated Files

• client/src/main.jsx — Added react-router-dom routing + navbar
• client/src/styles.css — Added responsive styles for new pages
• client/package.json — Added react-router-dom dependency

Testing

• All 6 new API endpoints verified
• Frontend production build passes (vite build)
• Progress tracking verified
• Doubt history CRUD verified
• Responsive layouts tested at 1050px and 640px breakpoints