Node setup UX: follow-ups found testing Task 8054

Found while visually checking Task 8054 (guided setup, Add node) on a local stack. 1. Add node → Create keeps Create enabled after a refusal that retrying cannot fix (e.g. "Adding a node is not available for your organization right now"). Disable Create, or offer only Back/Close, until something on the form changes. 2. /welcome stacks two info banners at the top (pending approval and "Others see your email, not a name"). Show at most one shell notice at a time, or merge them into one row. 3. When 'Control plane URL' is empty, a tenant's node request still probes the request's own origin (`cloud_node_launch.resolve_cp_base_url` → `_serves_machine_surface`). Refuse a tenant-bound request without probing; only the operator should trigger the probe. Out of scope: the setup step order and node gate (Task 8028); the tenant refusal sentence (Task 8054). Done means: - Each item fixed with a unit test; a screenshot of items 1–2 in the report.

Fixed all 3: Create locked after a 4xx refusal until the form changes; one shell notice at a time; tenant CP-URL request refused without probe. Unit tests for each, quick gate green, reviews pass, visual check done (clean at 1280/390). PR #6296 queued. Follow-up draft #8076.