feat: add premium calculator with theme toggle, animations, and history

[Pramod-Munnoli]

💎 Premium Calculator Upgrade

Enhanced the calculator UI and functionality with modern, interactive features:

• 🎨 Theme toggle (dark/light mode with smooth transitions)
• 📊 Scrollable in-display history with click-to-reuse
• 💾 Auto-save state using localStorage
• 🔢 Smart display with 12-digit precision and scientific notation
• ✨ Smooth animations and visual feedback effects
• 📱 Fully responsive layout across all devices
• 🔒 Improved error handling with shake effect on invalid inputs

🖼️ Screenshots

🔆 Before

🌙 After

🧪 Testing

Manually tested on:

• ✅ Chrome, Edge, Firefox
• ✅ Desktop, tablet, and mobile screen sizes
• ✅ Verified auto-save works on reload
• ✅ Keyboard inputs functional in both themes
• ✅ Animations and transitions smooth

📘 Notes

• Built using pure HTML, CSS, and vanilla JavaScript — no frameworks or CDNs.
• Works directly in the browser (no build tools required).

🔗 Related Work

Enhancement to base calculator implementation — builds upon the previous MVP (#161).

If not automatically marked, please add the hacktoberfest-accepted label. 🙏

[vercel]

@Pramod-Munnoli is attempting to deploy a commit to the hrishikesh-dalal's projects Team on Vercel.

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