Add HabitInsights analytics component

[ashvin2005]

**## 📋 Description

This PR adds a new HabitInsights component that provides users with a comprehensive analytics dashboard to visualize their habit tracking performance and progress.

✨ Features Added

📊 Key Metrics Dashboard

Four beautifully designed insight cards displaying:

• 🎯 Total Habits: Count of all tracked habits
• 🔥 Active Streaks: Number of habits with ongoing streaks
• 🏆 Best Streak: Longest streak achieved across all habits
• 📈 Average Completion: Overall completion rate percentage

⭐ Most Consistent Habit

• Highlights the habit with the highest completion rate
• Shows current streak and completion percentage
• Encourages user engagement through positive reinforcement

🎨 Design Features

• Responsive grid layout (mobile-first approach)
• Dark theme matching HabitFlow's aesthetic
• Color-coded cards for easy recognition
• Smooth hover transitions
• Lucide icons for visual clarity

🎯 Benefits

• Motivation: Visual feedback helps users stay motivated
• Progress Tracking: Quick overview of habit performance
• Data-Driven: Helps users identify their strongest habits
• User Engagement: Encourages continued use through insights
• Clean UI: Maintains HabitFlow's minimalist design philosophy

📝 Files Added

• frontend/src/components/HabitInsights.jsx - Main component
• frontend/src/components/README-HabitInsights.md - Complete documentation