Added count down

[devikrishna111]

📌 Description

This PR adds a 3-second countdown feature (3…2…1…GO) before the typing test starts and fixes the issue where the timer kept increasing incorrectly.
The countdown prevents the timer from starting immediately after clicking Start Test, giving users time to prepare before typing.

Additionally, the timer logic was cleaned to avoid multiple timer executions caused by duplicate functions.

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🔗 Related Issue

Closes: #39

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🛠 Changes Made

• Added 3-second countdown animation before the test begins
• Disabled typing input during countdown
• Timer now starts only after countdown finishes
• Fixed timer increasing continuously bug
• Removed duplicate update_timer() and check_result() functions
• Improved overall timer logic and UI feedback

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✅ Checklist

• [✅ ] I have tested my changes
• [✅ ] My code follows project guidelines
• [✅ ] I have linked the related issue