Surface credential source diagnostics / 显示凭据来源诊断

[SivanCola]

Summary

• Track the effective source for provider credentials, including environment variables, project .env, Reasonix credentials, home .env, and legacy credentials.
• Surface credential source labels in desktop settings and provider auth errors.
• Warn when saving a provider key would still be shadowed by an environment variable or project .env after restart.

Verification

• go test ./...
• go test . in the desktop module
• npm --prefix desktop/frontend run typecheck

Related: #4608
Related: #4401

[SivanCola]

@codex review

[chatgpt-codex-connector]

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