Machines › Nodes: polish seen while unifying "nothing

Found while visually checking Task 7422 (/nodes and a never-connected node's page). 1. /nodes Seen column: "None" (4 mono chars) spills left out of the 28px Seen slot, so it sits under the Version header. Widen the slot or right-align it so the word stays under its own header. 2. Overview of a never-connected self-hosted node: Last Seen reads "None" with no route forward. Link the value (or add a small action) to the host's install command, as Health's remedy tip already describes. 3. Overview of a never-connected node still offers Drain and Re-enrol. Hide or disable them with a reason until the node has connected once, since neither does anything yet. Out of scope: server logic, the Verdict's wording, the Load metric. Done means: - Screenshots of /nodes and of a never-connected node's Overview at desktop and 448px, showing each item. Required checks: ./dc check full

Merged #6056 (d7cbfab82): /nodes Seen centred+widened under its clock; never-connected node shows "None · Install" (re-issue → install command); Drain/Re-enrol disabled "Not connected yet". Quick gate + vitest green; visual check done at desktop/448px. Its deploy was superseded. Drafts filed: #7472 UI polish, #7521 review follow-ups (blocked by develop's duplicate alembic rev, #7502).