Add a pull request template with setup and validation checklist

[HarveySang]

Problem

Pull requests need a consistent checklist so contributors can show what they changed, what they tested, and whether Android device-level verification was possible.

Proposed scope

Add a lightweight PR template that keeps reviews focused without turning small contributions into heavy process.

Acceptance criteria

• Add .github/pull_request_template.md.
• Include server validation notes when relevant.
• Include Android build or device verification notes when relevant.
• Include a place for test notes and manual verification.
• Remind contributors to redact secrets, private screenshots, and sensitive logs.