Viewer: show node id and isolated-subtree pill in the detail header

[EYH0602]

Background

Gap analysis vs the deployed ara-hub viewer (https://www.agenticresearch.sh/ara/AmberLJC/ara-paperbench/artifacts/speedrun/nanogpt-speedrun).

Problem

1. The detail header renders only title + kind chip + support pill (crates/ara-viewer/src/detail.rs:423-437). The node id (N03) is never shown in the detail pane, making it hard to reference or cross-check a step against the map. The hub shows the mono id under the title.
2. Node.isolated (crates/ara-core/src/manifest.rs:276) only produces the isobox treatment in tree display mode. In graph mode and in the detail pane there is no isolated-subtree signal at all. The hub shows an iso pill in the detail header.

Proposal

• Render the node id in muted mono next to/under the detail title.
• When node.isolated (or the node belongs to an isolated subtree), render an isolated pill in the detail header meta row.

Acceptance

• Detail header shows the node id for every node
• Isolated nodes show an isolated pill in the detail header
• Unit tests in detail.rs

[EYH0602]

Fixed in #65.