fix(cli): display resolved auth path

[upsetbit]

Closes #76.

Summary

• resolve the login help/final token path through paths.AuthPath()
• have SaveAuth write via paths.AuthPath() directly
• reword CLI auth comments away from hardcoded ~/.config/prosa/auth.json

Local validation

• go test ./internal/cli/...
• devbox run -- just lint
• devbox run -- just test-race

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