Refactor pull request template for better structure and UI guidance

[vitormattos]

Is your feature request related to a problem? Please describe.

Our current Pull Request template is helpful, especially with the Codespaces testing guide. However, the separation between frontend (UI) and backend (API) is not clear, and the structure could be more visually organized for contributors.

We would like to redesign the template to:

• Retain our Codespaces/"How to test" instructions;
• Clearly separate frontend (UI) and backend (API) sections;
• Improve visual layout and clarity, inspired by the clean approach in this template example.

Describe the solution you'd like

• Review the current pull request template;
• Design a new template that:
  • Keeps the Codespaces/How to Test section;
  • Clearly separates UI (frontend) and backend/API changes;
  • Adds a UI checklist, including before/after screenshots for visual changes;
  • Uses simple, easy-to-read formatting;
• Update .github/pull_request_template.md with the new design;
• In your Pull Request, include:
  • The new template file;
  • In the PR description, a filled-out example of the new template (especially the UI checklist and before/after images).

Describe alternatives you've considered

• Use ideas from the example template to improve layout and clarity or other pull request template that you consider as good to solve the problem of this issue;

Additional context

• If you are unsure how to structure the template, start by looking at the example template and adapt it for LibreSign.
• For UI changes, you can use any image (even a placeholder) for before/after. Markdown tables are fine.
• If you have questions, feel free to ask in this issue or the discussions tab.
• Give a ⭐️ star to this repository

---

This task is suitable for first-time contributors!