docs: add pull request template for contributors

.github/pull_request_template.md

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+## Description
+
+<!-- What does this PR do? Keep it concise. -->
+
+## Type
+
+- [ ] `feat` — New feature
+- [ ] `fix` — Bug fix
+- [ ] `docs` — Documentation only
+- [ ] `refactor` — Code change that neither fixes a bug nor adds a feature
+- [ ] `style` — Formatting, CSS, UI tweaks
+- [ ] `perf` — Performance improvement
+- [ ] `ci` — CI/CD changes
+- [ ] `chore` — Maintenance / tooling
+
+## Checklist
+
+- [ ] `npm run lint` passes with 0 errors
+- [ ] `npm run build` succeeds
+- [ ] Tested in browser (dark + light theme)
+- [ ] No hardcoded secrets or credentials
+- [ ] Commit messages follow [Conventional Commits](https://www.conventionalcommits.org/)
+
+## Screenshots
+
+<!-- If this changes the UI, add before/after screenshots. Delete this section otherwise. -->
+
+## Related Issues
+
+<!-- Closes #123, Fixes #456 — or delete this section -->