docs: Add standardized Pull Request template

[ayushsri-dev]

Context

The Arvio repository currently lacks a standardized Pull Request template. When contributors open pull requests on GitHub, they are presented with an empty text box.

Problem

Without structured guidance during PR creation:

1. Contributors frequently omit linked issues (Closes #123), leaving resolved issues open after PRs are merged.
2. Maintainers lack clarity on which device form factor (Android TV, Phone/Tablet, iOS) or build variant (sideload vs play) was tested.
3. UI and layout changes are often submitted without visual screenshots or video recordings.
4. There is no pre-submission checklist reinforcing compliance with Arvio's Content & Source Policy (ensuring no credentials or copyrighted media are included).

Proposed Solution

Add a clean, targeted .github/PULL_REQUEST_TEMPLATE.md file to standardize PR submissions across the project.

Key Template Sections

• Summary: Concise description of changes.
• Related Issue: Auto-closing keyword prompt (Closes #).
• Type of Change: Categorization checkboxes (Bug fix, Feature, UI/UX, Refactor, Docs).
• Platforms & Variants Tested: Explicit checkboxes for Android TV, Mobile/Tablet, iOS, Sideload/Play build variants.
• Screenshots / Demos: Table placeholder for Before/After media for visual updates.
• Contributor Checklist: Verification of local build compilation and Content Policy compliance.

Expected Benefits

• Faster and more consistent code reviews for maintainers.
• Automated closing of resolved issues via proper issue linkage.
• Clear confirmation of multi-platform test coverage (Android TV remote vs. touch UI).
• Reduced review overhead and back-and-forth communication.
• Better community standards and repository health

Acceptance Criteria

• .github/PULL_REQUEST_TEMPLATE.md is created in the repository.
• Opening a new PR on GitHub automatically populates the template.
• Template contains clear section headers for summary, issue linking, change classification, device/variant testing, UI evidence, and policy checklist.
• No changes are made to existing CI workflows or source code.

[ayushsri-dev]

/claim