Automate pre-release test-APK flow (Closes #5)

[ShubhamJ010]

Summary

Implements #5 — replaces the ad-hoc manual pre-release with an automated pre-release test-APK flow as standard CI/CD practice.

• .github/workflows/build-apk.yml now publishes a debug-signed test APK as a pre-release on every push to feature/**/fix/** and every PR to main, in addition to the pipeline artifact.
• Pre-release tags are keyed to the source: test-<branch-slug> for branch pushes, pr-<number> for PRs — updated in place via allowUpdates, so old test builds don't accumulate (clutter-free).
• Keeps the upload-artifact step so the APK is also downloadable straight from the run.
• Signing is debug (CI has no keystore); the pre-release body states it's debug-signed for sideloading. An optional release-signed build via repo secrets can be added later per the issue.

Depends on #4

This PR is stacked on feature/issue-2-lower-minsdk-api29 (PR #4) and must be merged only after #4 is merged/closed first. Once #4 lands on main, rebase this branch onto main and switch this PR's base to main, then merge to close #5.

Closes #5