Implement VersionInfo and enhance project documentation and workflows

[FesterHead]

📝 Description

This PR merges the 1.0.3 release from develop into main, establishing formal contributor guidelines, GitHub Issue forms, Pull Request templates, and CI build security hardening.

Summary of Changes in v1.0.3:

• Contributor Guidelines (CONTRIBUTING.md): Added official contribution guidelines detailing MP3 playback constraints, minimalist UI philosophy, $O(1)$ primitive array memory footprint, develop ➔ main branching rules, MediaStore/SAF path resolution, smart ReplayGain, and KDoc/testing standards.
• GitHub Issue Forms (.github/ISSUE_TEMPLATE/): Added structured bug_report.yml, feature_request.yml (with mandatory scope alignment checkboxes), and config.yml.
• Pull Request Template (.github/PULL_REQUEST_TEMPLATE.md): Created standardized PR template and verification checklist.
• CI Hardening (.github/workflows/pr-build.yml): Added assembleDebug verification and isolated release keystore signing behind HAS_KEYSTORE secret presence checks to support fork PRs safely.
• Project Status & Linting: Updated README.md disclaimer section to indicate PRs aligning with project goals may be accepted, and added project-level .markdownlint.json / .vscode/settings.json rules.
• Version Bump: Updated version.properties and VersionInfo.kt to version 1.0.3 (Build 4) and updated CHANGELOG.md.

Fixes / Closes N/A

🌌 Scope & Architecture Checklist

• MP3 Playback Only: Changes are strictly for local MP3 audio files and M3U/M3U8 playlists.
• Minimalist UX: Maintains single Play/Pause control; no seek bars or skip/navigation buttons added.
• Linear Memory Footprint ($O(1)$): Low memory footprint relative to queue size maintained using primitive arrays (LongArray/IntArray) and Room sliding cursors.
• Gapless & Audio Focus: Media3 gapless playback and Android Audio Focus handling preserved.

🧪 Testing Checklist

• I have executed ./gradlew test locally and all unit tests pass.
• New classes, interfaces, methods, and functions include full KDoc documentation (@param, @return).
• No raw absolute file paths or unhandled exceptions introduced.
• CHANGELOG.md updated under ## [Unreleased] (if applicable).

[FesterHead]

Closing pull request as status checks are foobared.

[FesterHead]

Removed status check requirement. I guess we can add it later, who knows, status check naming is not very intuitive or straightforward.