feat: Add Easter Egg detection for optimized prompts (Issue #27)

[codewithdhruba01]

This pull request adds an Easter Egg feature to the sample-app/app.js file as requested in [Issue #27](#27).

✅ What was added:

• Prompt input from the user.
• Call to the AI model using the user’s prompt.
• If the user enters a highly optimized prompt (e.g., "Write me a poem about a time-traveling JavaScript developer"), the application detects it and displays a hidden congratulatory Easter Egg message.

✅ Files modified:

• lessons/03-prompt-engineering/sample-app/app.js

✅ Testing:

• Manually tested by entering various prompts.
• Hidden message successfully appears when an optimized prompt is used.

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> Enter your prompt:
Write me a poem about a time-traveling JavaScript developer

AI Response:
[Poem generated...]

Easter Egg Unlocked! You discovered the hidden poem! 

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Final Checklist :

• Code tested locally.
• Message matches the issue requirement.
• No unnecessary changes added.

[codewithdhruba01]

Please review the changes....