fix: fail cleanly on incompatible platforms

[decofe]

Summary

• reject incompatible foreign-platform overrides before network retries or downloads, while preserving Linux-to-Alpine selection
• cancel the background self-update check when installation fails so error output is immediate and unambiguous

Testing

• cargo test --all-targets -- --test-threads=1
• cargo +nightly clippy --all-targets --all-features -- -D warnings
• cargo +nightly fmt --all -- --check
• manually verified a Linux-to-Windows override exits immediately without an updater success message

Prompted by: @0xalpharush