[Refactor] Derive onboarding order from one canonical flow

[roomote-roomote]

​Created by Roomote. View the task or mention @roomote-roomote for follow-up asks.

What changed

• Kept the setup step definitions as the single canonical flow.
• Derived the email/password sequence through a typed policy that moves only the communication setup block after source control.
• Added focused coverage that preserves both complete step orders and verifies they contain the same canonical steps.

Why this change was made

PR #1331 introduced a second complete setup sequence to support a different provider order for email/password users. Keeping two full arrays made future step additions and renames vulnerable to auth-mode-specific drift.

Impact

There is no intended user-facing behavior change. Email/password users retain the reordered provider flow, communication-auth users retain the canonical flow, and future setup steps now enter both modes from one definition.

[roomote-community]

No code issues found. See task

_{Reviewed bf7bc35}