feat: add calculation chaining, emoji interface, and error handling t…

[Alvi24-hub]

📝 Description

This PR adds three major enhancements to the calculator:

1. Calculation Chaining — After getting a result, users can choose to chain with new numbers (e.g., 10 + 5 = 15, then 15 × 2 = 30)
2. New Calculation Option — Start fresh calculation without exiting the program
3. Emoji Interface — Added emojis throughout menu and outputs
4. Validates inputs for chaining and new calculation prompts
5. Fixed exit not stopping the program properly

🔗 Linked Issue

Closes #530

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📋 Contribution Checklist

• My code strictly adheres to the project guidelines (Standard library only, procedural code, clean spacing).
• I have verified that my files are placed in the correct directory.
• I have tested my changes thoroughly on my local machine.
• I have included interactive emojis and clean console/UI outputs.
• GSSoC 2026: I have been formally assigned to this issue and noted it above.

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[github-actions]

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