fix(desktop): update check follows upstream proxy settings

[GcsSloop]

Summary

• make desktop update check read /settings/app upstream proxy config
• align updater check path with system/direct/manual proxy modes
• support manual proxy credentials injection for updater checks
• add updater proxy parsing tests in tauri desktop unit tests
• bump version to v1.3.20

Verification

• cargo test --manifest-path desktop/src-tauri/Cargo.toml updater_proxy_config -- --nocapture
• cargo test --manifest-path desktop/src-tauri/Cargo.toml update_check_timeout_returns_error_for_stalled_future -- --nocapture