Fix welcome wizard unavailable paths

Found while screenshotting /welcome (Task 8560) on an org with no GitHub app and no node offering. 1. Connect Git with no GitHub app shows only "Ask the platform operator to set up GitHub." Keep "Other options" (GitLab, Bitbucket, Azure DevOps, token) available there, so the step can still be finished without GitHub. 2. Three waiting steps each repeat "Waiting for your node". Say it once (under Your node, or as one line above the waiting group) and leave the waiting steps without a note. 3. Your node and Connect Git both tell the user to ask an operator, in different words and with different actions ("Contact support" vs. plain text). Use one wording and one action for "only an operator can fix this". 4. When the GitHub app is missing, a returned `?git_connect=error` banner (for example "declined at GitHub") sits above the operator ask, which is confusing. When there's no app, show only the operator-fixable reasons (`not_configured`, `wrong_app_kind`). Out of scope: the server's setup-step model (`GET /api/org/setup`), sign-up pages, and node provisioning. Done means: - Each item is fixed or rejected, with one line in the PR. - WelcomePage vitest covers 1, 2 and 4. - Screenshots of /welcome at 375 px and 1280 px, light and dark.

Fixed all 4 /welcome items: Other options kept with no GitHub app, node gate said once, one operator ask (Contact support), only operator-fixable return errors. Vitest + quick gate green; visual check done (375/1280, light/dark). PR #6503 queued in merge lane. Follow-up draft #8649. Memory: none applied.