vscode: account/cloud step in first-run walkthrough (IDE-featured, marketplace-available)

[amrmelsayed]

Depends on the AuthenticationProvider + cloud UI issues. Adds an optional "Create a Codev account / Connect to Cloud" step to the existing getting-started walkthrough.

Scope

1. Walkthrough step with sign-in/sign-up action (drives the AuthenticationProvider flow) + a one-line value prop (remote access to your builders).
2. Placement: FEATURED when `codev.ideMode` is true (in the IDE this walkthrough is the product's only onboarding); present-but-quiet for marketplace users.
3. Skippable, never blocking: the local-only experience remains fully functional without an account. Step completes/hides when a session exists.

Acceptance

• IDE-mode first run offers account creation without demanding it; marketplace first activation shows no new prompts; signed-in users never see the step.