fix: add max length validation on signature text input

[pericharlabindhumadhavi-data]

🔀 Pull Request

📌 Issue Reference

Closes #272

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📝 Summary

Problem

The PDF Sign feature accepted signature text of any length without validation.

This caused:

• Users to submit excessively long signature text.
• Potential rendering and processing issues when generating signatures.
• No consistent input validation between frontend and backend.

Solution

This PR introduces a maximum signature length limit of 500 characters and enforces the restriction on both the frontend and backend.

The frontend prevents users from entering more than 500 characters, while the backend validates incoming requests and rejects oversized inputs with an appropriate error message.

Changes Made

• Added backend validation for signature text length.
• Added a 500-character maximum limit.
• Added frontend maxLength={500} validation.
• Added clear error handling for oversized signature inputs.

✅ Checklist

• My code follows the project's coding conventions
• I have tested all impacted features
• I have updated or added necessary documentation

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🔗 Related Issues / PRs

• Issue [BUG] PDF Sign — no maximum length validation on signature text input #272

🏅 Open Source Program Participation

Program Name: GSSoC 2026 (GirlScript Summer of Code)

💬 Additional Notes

Testing Performed

• Verified signatures under 500 characters are accepted.
• Verified signatures exceeding 500 characters are rejected.
• Verified frontend prevents entering more than 500 characters.
• Verified backend validation works even when requests bypass the frontend.

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