Add node dialog and Brains: polish seen while testing Task 8021

Found while checking Task 8021 on a live stack. Each item is a short requirement; decline any that no longer applies. 1. Add node dialog, step How: "Jaah Cloud" carries an "Unavailable" chip, yet an active Org picker ("Hosts a tenant org's machine. Pick the org:") sits under it. Hide or disable the picker while its option is unavailable, so the reader is not invited to fill a field that leads nowhere. 2. Brains › Add node (`/nodes?add=1`, `components/addNode/deepLink.ts`): the dialog opens with the Organization the viewer's default, not the account's org. Carry the account's org on the link (the dialog already takes `initialOrgId`) so an operator adding a node for a tenant's account lands on that org. Out of scope: the Add node flow's later steps; the Brains row layout. Done means: - Each item shipped or declined with a one-line reason in the PR, with web unit tests where behaviour changed. - Screenshots at 1280px and 390px of each changed surface. Required checks: ./dc check full

Both items shipped in #6286 (merged 7f705c5, lane full gate green). Jaah Cloud has no Unavailable chip over its org picker. The Brains Add node link carries the account's org, and the dialog names it and offers Act as. Screenshots taken at 1280/390px. Follow-up draft #8063 filed.