fix: add unsaved changes warning

[ahuseyn]

Description

This PR adds a beforeUnload event handler to warn users about unsaved changes.

Related Issue

• [Bug](previews): Loosing config changes #195

Dependant PRs

N/A

Type of Change

• ✅ Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactoring (no functional changes)
• 📄 Example update (no functional changes)
• 📝 Documentation update
• 🔍 Performance improvement
• 🧪 Test update

How Has This Been Tested?

Manual, e2e

Screenshots

Checklist

• I have read the CONTRIBUTING document
• My code follows the project's coding standards
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• Any dependent changes have been highlighted, merged or published

[github-actions]

ℹ️ Download the latest hwp-previews plugin zip from this PR
(See the 'Artifacts' section at the bottom)

[colinmurphy]

@ahuseyn Nice :) 🚀