docs: add Google Play Store badge and production rollout details

[GiZano]

Description

This PR updates the project documentation to reflect that ClimbStack is officially in production on the Google Play Store:

• README.md:
  • Adds the official Get it on Google Play badge in the header directly beneath the open-source tagline.
  • Adds a dedicated 📥 Install from Google Play installation section with direct link and package identifier (com.gizano.levelup).
  • Includes Google Play badge in the shields section.
• ROADMAP.md:
  • Marks Phase 3 (Automation and CI/CD) as [COMPLETED ✅].
  • Introduces Phase 4 (Production Release & Distribution [LIVE 🚀]) documenting the Google Play Store launch and package ID.

Verification

• Verified Markdown rendering and links.
• Verified package URL: https://play.google.com/store/apps/details?id=com.gizano.levelup

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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