feat(BMI calculator): add unit toggle, localStorage with list & chart history, improved UI/UX

[nur-hasin]

Summary

This PR implements all the core features for the BMI Calculator. The implementation includes comprehensive functionality for calculating, tracking, and visualizing Body Mass Index data with history saving, unit toggling, input validation states, and UI/UX improvements.

Fixes #13

Changes Made

• History & Visualization - Implemented localStorage-based history (10-entry limit) with list view and Chart.js line chart for BMI trend analysis
• Unit System - Added metric/imperial toggle with real-time conversion and dynamic placeholder updates
• Validation - Built real-time input validation with visual feedback (success/error states) and submit button state management
• UI/UX - Created modern glassmorphism design with gradient backgrounds, custom input arrows, smooth animations, and mobile-responsive layout
• Architecture - Developed modular JavaScript class with event-driven interactions, error handling, and Chart.js integration

Checklist

• Linked issue: Feat: Add BMI history and UI improvements #13
• Ran locally without console errors
• Focused scope (kept PR small)
• Updated docs or TODO comments if needed

Video

Untitled.video.-.Made.with.Clipchamp.4.mp4

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