fix(auth): implement PKCE for Google Auth and sync latest upstream notifications

[ranjank2alpha]

Summary

• Implements PKCE (code_challenge and code_verifier with S256 method) for native Google OAuth2 login flow in google_auth.rs.
• Adds explicit HTTP status verification (status().is_success()) before parsing OAuth token exchange JSON responses to prevent unwrap crashes on 4xx responses.
• Merges latest upstream main updates and resolves notification sound/active target check conflicts in use-feed-desktop-notifications.ts.

Testing

• Code audit completed across Rust and TypeScript network boundary handlers.
• Verified PKCE parameter generation and error body logging.