Fix v0.3.0 release without signing secrets

[AugustoOM]

Summary

• allow the v0.3.0 desktop release to build when signing secrets are not configured
• preserve signed Windows and notarized macOS builds when complete credentials are available
• reject partially configured credentials with a clear error
• update the release documentation

Validation

• release versions agree at 0.3.0
• formatting passes
• lint passes
• 214 tests pass
• type checking passes

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