Add OAuth authorization code flow to console app

[Copilot]

The console app had no way to perform the initial OAuth authorization — users had to obtain tokens out-of-band. This adds a first-class 'o' (authorize) option that drives the full authorization code flow.

Changes

• New 'o' option in the startup token prompt alongside existing 'a' (access token) and 'r' (refresh token) paths
• AuthorizeAsync method that:
  • Constructs the Enphase auth URL with response_type=code, client_id, and the default redirect_uri (https://api.enphaseenergy.com/oauth/redirect_uri)
  • Opens the URL in the default system browser via Process.Start(..., UseShellExecute = true); degrades gracefully if the browser can't be launched
  • Prompts the user to paste the authorization code from the redirect URL
  • POSTs to /oauth/token with grant_type=authorization_code using Basic auth (base64 client_id:client_secret)
  • Prints the returned refresh token for the user to save for future runs
  • Returns the access_token to initialize the API client

Do you have an access token or refresh token, or do you need to authorize?
Enter 'a' for access, 'r' for refresh, or 'o' to authorize:

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