release: prepare v0.5.0-beta.2

[karan68]

Release

• bump desktop and Android to 0.5.0-beta.2
• route beta desktops through the beta-channel updater manifest
• add signed Android APK build, signer verification, provenance, and checksums
• add verified unsigned Windows beta workflow with updater signatures and installer smoke tests
• normalize Rust formatting so the repository-wide formatting gate passes

Local validation

• Rust: 237 passed, 2 ignored, 0 failed
• frontend type-check and production build passed
• signed Android R8 APK built and verified (versionCode=2)
• Windows release executable built with embedded 0.5.0-beta.2
• release policy tests and PowerShell parser checks passed
• no credential-like values in staged patch