Fix Fibonacci input visibility and validation issues(#489)

[siri-004]

📝 Description

This PR fixes usability and validation issues in the Fibonacci Series generator modal

✅ Changes Made

• Fixed invisible input text issue in light mode by improving input field styling and contrast.
• Added proper validation for empty and invalid inputs.
• Prevented Fibonacci sequence generation when the input is empty or less than or equal to 0.
• Added user-friendly validation error messages.
• Removed unintended default Fibonacci generation behavior.

🔗 Linked Issue

Closes #489

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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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