fix(client): say when the account's node is elsewhere, and let go of it

Signing in on a desktop links this machine's node to the account, but never
over a key already linked (that would cut off the user's other machine) nor a
node set up for another account. On a real install both left the node reading
"Running" while the hub refused it in a loop, and nothing said why. And the
only way to unlink was the linked machine's own Node page, of no use once
that machine is gone.

- The Node page works out, from the node and the hub each time it looks,
  whether this node can serve the signed-in account (nodeLinkProblem), and
  says why not: "Link this node instead" (asked first) puts this node's key
  on the account; "Use this node for my account" switches a node set up for
  another account through node:start, which takeOver now lets past a node
  that answers "running". The first version went through node:start for
  both, and clicking it on a real install did nothing: a node signed in
  before the account was linked elsewhere answers "running".
- The Profile page unlinks the account's node (DELETE /v1/users/me/node_key),
  from any machine.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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