#360 feat(desktop): add OAuth authentication for desktop app

Summary

• Implements OAuth authentication flow for the desktop app using PKCE
• Adds token refresh mechanism with 30-day refresh tokens for persistent sessions
• Creates intermediate web callback page for desktop auth redirects
• Adds script to register superset-dev:// deep links in development on macOS

Changes

• API: Token exchange and refresh endpoints for desktop auth
• Desktop: Auth service with PKCE, token storage, auto-refresh logic
• Desktop: AuthButton component in top bar
• Web: /auth/desktop/callback page for OAuth redirects
• Web: /api/auth/desktop/[provider] endpoint to initiate OAuth flow
• Desktop: Dev protocol handler script for macOS deep links

Test plan

• Sign in via Google OAuth from desktop app
• Verify token refresh works (tokens auto-refresh before expiry)
• Verify session persists across app restarts
• Test sign out clears session
• Test deep links work in development on macOS

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Desktop app: sign-in with GitHub and Google, deep-link callback handling, and automatic session/token management.
  • New Sign‑In screen, Superset logo, and Account Settings with profile display and sign-out.
  • Desktop ↔ Web auth flow with one‑click redirect to the app and improved single‑instance deep‑link handling.
• Chores
  • Added cross‑platform protocol registration and configuration improvements for desktop builds.

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