feat: added entry point and app arg to force onboarding flow

[davidejensen]

Pull Request Description

What does this PR change?

This PR adds an entry point checkmark and an app arg that force the onboarding flow as if we were new users.
This will simplify testing the flow instead of creating a new account every time.
Keep in mind that this is not 100% as if we were starting with a new account as the profile is not erased, it just forces on client side the onboarding flow in the initial loading

Test Instructions

Prerequisites

Test Steps

1. Launch the client with a new user and check that the onboarding flow is working correctly

2. Launch the client with an old user and verify that the onboarding flow doesn't happen

3. Launch the client with an old user with the parameter --force-onboarding and verify that even if the account is old the onboarding flow happens

Additional Testing Notes

• Note any edge cases to verify
• Mention specific areas that need careful testing
• List known limitations or potential issues

Quality Checklist

• Changes have been tested locally
• Documentation has been updated (if required)
• Performance impact has been considered
• For SDK features: Test scene is included

Code Review Reference

Please review our Code Review Standards before submitting.

[github-actions]

Windows and Mac build successful in Unity Cloud! You can find a link to the downloadable artifact below.

Name	Link
Commit	ce3b71a
Logs	https://github.com/decentraland/unity-explorer/actions/runs/18315222197
Download Windows	https://github.com/decentraland/unity-explorer/suites/47085538725/artifacts/4204734164
Download Windows S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/feat/force-onboarding/pr-14688-ce3b71a/Decentraland_windows64.zip
Download Mac	https://github.com/decentraland/unity-explorer/suites/47085538725/artifacts/4204487672
Download Mac S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/feat/force-onboarding/pr-14688-ce3b71a/Decentraland_macos.zip
Built on	2025-10-07T14:48:44Z

[m3taphysics]

This is blocked until we re-enable the onboarding flow.