feat(frontend): add loading state and disable buttons during active requests

[Arijit429]

🚀 Summary

This PR improves frontend production readiness by adding visible loading states and disabling action buttons while API requests are in progress.

The goal is to provide clearer user feedback and prevent repeated interactions during active request flows.

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✨ What Changed

Updated the frontend request workflows in:

frontend/app.js
frontend/index.html

Added visible loading state handling

For both primary user workflows:

• Create Template
• Fill Form

the submit buttons now:

• become disabled while the request is active
• show temporary loading text
• restore their original state after completion

Examples:

Create Template → Creating...
Fill Form → Submitting...

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💡 Why This Helps

Previously, requests were internally protected from duplicate submissions, but there was no clear visual feedback to the user.

This update improves:

• user experience clarity
• professional UI behavior
• confidence during longer-running API calls
• prevention of repeated clicks caused by uncertainty

This pairs well with the earlier duplicate-request protection logic.

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🎯 Production Impact

This makes the frontend feel significantly more production-ready by improving request-flow transparency and reducing accidental repeated interactions.

It strengthens the overall user workflow reliability for real-world usage.