dev: Added contributor guidelines and updated the PR template

CONTRIBUTING.md

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+# Contributing to ElegantFin
+
+Thank you for your interest in contributing to **ElegantFin**, a custom theme for the Jellyfin Web UI. To keep the project clean, maintainable, and compatible across devices, please follow these guidelines when contributing.
+
+---
+
+## 🧩 Pull Request Guidelines
+
+- **Keep PRs focused.** Each pull request should address a single feature, bugfix, or improvement. Avoid bundling unrelated changes.
+
+- **Write clear PR descriptions.** Explain what you changed and why. This helps reviewers and future contributors understand the purpose and reasoning behind the change.
+
+- **Provide screenshots.** Include before/after screenshots for any visual changes, along with a short description of what each image shows.
+
+- **Comment your code.** If you’re adding new CSS rules or modifying complex ones, include inline comments where needed to describe intent.
+
+- **Don't forget responsiveness.** ElegantFin runs on phones, tablets, TVs, and desktops. Ensure your changes are tested across viewports and don't break layouts or interactions on other platforms.
+
+- **Use consistent units.** Follow the theme’s existing use of relative units such as `em` instead of absolute units like `px`.
+
+- **Avoid `!important`.** Only use it when it is absolutely necessary and no alternative exists.
+
+- **Minimize media queries.** Only add new media queries if there is no simpler or existing solution that achieves the same result.
+
+---
+
+## 💬 Propose First for Major Changes
+
+If you are planning to:
+- change the overall layout or structure of key components,
+- modify the core color scheme,
+- or introduce new UI behavioral patterns,
+
+Please open a GitHub issue first to discuss the proposal with maintainers before submitting a PR. This helps avoid duplicate work and ensures alignment with the project's goals.
+
+---
+
+## 📁 File Structure and Naming
+
+- Follow the existing CSS naming conventions and structure.
+- When adding new sections, group related styles logically and use comments to identify the purpose of the block.
+
+---
+
+## 🤝 Communication
+
+- Be respectful, constructive, and collaborative.
+- Be open to feedback and willing to revise your work if needed.
+
+---
+
+We appreciate your help in making ElegantFin better for everyone. Thanks for contributing! 🙌