Ignore updates for unversioned builds

[robertmonka]

🧢 Changes

• Treats missing, empty, or 0.0.0 compile-time VERSION values as unversioned development builds.
• Skips remote update checks when the build does not have a real release/nightly version.
• Ignores cached update results for unversioned builds instead of comparing them against a 0.0.0 fallback.
• Changes the default update status version from 0.0.0 to dev.
• Updates but update check human output to explain that update checks are only available for versioned release and nightly builds.
• Adds focused coverage for version-aware cached update handling and current-version request behavior.

☕️ Reasoning

Local and development builds can be compiled without a real VERSION. Previously those builds fell back to 0.0.0, which made update checks compare a dev binary against published release data and could surface a misleading available update.

This makes unversioned builds opt out of release-channel update comparisons entirely, while keeping normal update behavior for packaged release and nightly builds that do include a real compile-time version.

🧪 Testing

• Added/updated Rust tests for cached update handling with and without a real build version.
• Not run locally while updating this PR description.