feat(auth): allow configurable redirect URI in remote/manual OAuth flow

[salmonumbrella]

Summary

• add --redirect-uri to gog auth add for manual/remote OAuth flows
• thread redirect override through googleauth.AuthorizeOptions
• use redirect override for remote step 1 URL generation and manual/code exchange when needed
• preserve state validation behavior and add tests for override/reuse/precedence/invalid input

Testing

• go test ./internal/googleauth ./internal/cmd
• go test ./...

Closes #356

[steipete]

Landed via temp rebase onto main.

• Gate: go test ./internal/cmd ./internal/googleauth -run 'TestAuthAddCmd_RemoteStep1_|TestAuthAddCmd_RemoteStep2_PassesRedirectURI|TestManualAuthURL_|TestAuthorize_Manual_|TestNormalizeRedirectURI|TestAuthorize_InvalidRedirectURI' -count=1, make ci
• Land commit: c88bbdb
• Merge commit: 70c751c4001c0207adaf265b49a61ff65e9b2c4c

Fixups included:

• preserve --redirect-uri in the remote step-2 replay guidance
• changelog entry
• extra regression coverage around override propagation and redirect/state behavior

Thanks @salmonumbrella!