Add download validation and domain reachability checks

[EstusFlask]

Summary

• Show a green completion check on Mods and Saves when every resource is downloaded, or when every remaining missing resource has been confirmed invalid.
• Automatically validate incomplete downloads by checking domain reachability before checking individual resource URLs.
• Distinguish unreachable-domain failures from invalid-resource failures in the result dialogs.
• Add a Settings > Network action that checks every unique domain referenced by Mods and Saves, including excluded domains.
• Persist invalid URL results and invalidate the cache when the source JSON timestamp changes.

Behavior

• Completion indicators are shown in both grid and list views; Saved Objects retain their existing behavior.
• Entries with zero resources are considered complete.
• Post-download validation is wired into single, asset-level, backup, and bulk download flows.
• Any HTTP response counts as a reachable domain; only transport-level failures mark a domain unreachable.
• Invalid URLs are persisted only after all relevant domains are reachable, avoiding false invalid results caused by network conditions.

Testing

• flutter analyze --no-pub
• flutter build windows --release --no-pub
• Smoke-tested the generated Windows x64 release executable

Screenshots

Completion indicators, invalid-resource results, domain reachability results, and the new Network settings action: