Fix macOS Apple authentication and onboarding form contrast

[artembaechka]

Summary

• keep the complete compatible X-Apple-* identity returned by anisette-v3 instead of mixing it with locally generated device metadata
• preserve the selected trusted-device or SMS method when GSA authentication resumes after 2FA
• use the accepted com.apple.akd/1.0 GSA client identity and retry transient 502/503/504 responses
• report HTTP status, Content-Type, and a bounded response preview when Apple returns a non-plist response
• make Apple ID, password, verification-code, and 2FA controls readable across macOS themes
• add regression coverage for anisette identity, 2FA recursion, GSA diagnostics, client identity, and retries

The anisette behavior follows the fix proposed in MatthewKuKanich/FindMyFlipper#144.

Validation

• Backend/venv/bin/python3 -m pytest Backend/tests -q: 37 passed
• swift build -c release: passed
• verified the bundled GUI Apple Access flow against anisette-v3

No generated .keys files, Apple credentials, authentication tokens, or local runtime data are included.