ci: separate Android mobile releases from desktop releases

[wgqqqqq]

Summary

• add an Android-only mobile release workflow triggered by mobile-v* tags
• publish mobile APKs as separate GitHub Releases with SHA256 checksums
• use debug signing automatically for fork validation builds while requiring official signing secrets in GCWing/OpenBitFun
• remove duplicate Linux package globs from desktop release staging
• document the mobile release channel and link it from the project README

Validation

• pnpm run check:github-config
• pnpm run check:repo-hygiene
• local :app:assembleRelease completed successfully
• fork GitHub Actions run 38021058465 completed successfully
• fork test release: https://github.com/wgqqqqq/BitFun/releases/tag/mobile-v1.0.0