In loadFullNode's catch path:
- Update .node-full-title to 'Node not found — <prefix>…' on 404 (or
'Failed to load node' on other errors), instead of leaving 'Loading…'.
- Replace the body's bare text with a card showing the requested pubkey,
a friendly explanation, a 'Back to Nodes' link (href='#/nodes'), and a
'Try again' button that re-invokes loadFullNode.
Fixes the symptom from issue #1150 (header title stuck on 'Loading…')
and adds the affordances called out in the recommended fix (back link
+ retry).
Fixes#1150