[Fix] Back missing on setup Create GitHub App when steps are skipped

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

Related issue

Follow-up after #546 was merged — Back was missing on setup Create GitHub App.

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

What changed

• Always wires Back on the setup Create GitHub App / source-control config step.
• When earlier steps are skipped or deep-linked (no history), Back falls through to the source-control provider chooser — matching Slack’s create-app Back behavior.
• Covers that with a unit test on the GitHub create screen.

How it was tested

• Targeted web vitest for StepSourceControlConfig and setup flow hooks (58 passed)
• Pre-push (check-types:fast, residual lint, knip)
• Browser proof of the live setup step was blocked here (local setup already complete redirects off /setup)

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

[roomote-roomote]

No code issues found. See task

_{Reviewed bbc87ae}