Migrate desktop auth from dedicated Cloud Run to Python backend

[beastoin]

Context

The desktop macOS app uses a dedicated omi-desktop-auth Cloud Run service (https://omi-desktop-auth-208440318997.us-central1.run.app/) for OAuth authentication. However, the Python backend (api.omi.me) already has identical /v1/auth/* endpoints:

• GET /v1/auth/authorize — generate OAuth URL
• GET /v1/auth/callback/google — Google OAuth callback
• POST /v1/auth/callback/apple — Apple OAuth callback
• POST /v1/auth/token — exchange custom token for Firebase ID token

The dedicated service is redundant and adds unnecessary infrastructure to maintain.

Change

Update AuthService.swift to use the main backend URL (OMI_API_URL) instead of the hardcoded omi-desktop-auth Cloud Run URL.

File: desktop/Desktop/Sources/AuthService.swift:47

// Before
private let apiBaseURL: String = "https://omi-desktop-auth-208440318997.us-central1.run.app/"

// After — use the main backend URL from environment

Acceptance Criteria

• AuthService.swift reads the auth base URL from OMI_API_URL (or the shared config)
• No hardcoded reference to omi-desktop-auth Cloud Run service remains
• OAuth sign-in (Google and/or Apple) works end-to-end against api.omi.me
• Live test evidence from Mac Mini showing successful sign-in flow

Part of

Parent issue: #5302 (Migrate desktop macOS app from Rust backend to Python backend)

by AI for @beastoin