refactor(node): JSON-only control API, Node page absorbs the admin dashboard

Remove the node daemon's server-rendered admin UI (GET / and /audit, the
_render_* helpers and inline templates) and the `meshbay-node ui` CLI verb.
The loopback control API stays; it is now JSON only, ruff-clean, and 453
lines (was 1074). Also drop three never-wired endpoints (/api/config,
/api/chat/history, /ws/chat, plus broadcast_chat) and the pointless
18000/tcp firewall profiles.

The desktop client's Node page (static/node-page.js) takes over what the
dashboard showed, reorganised into six tabs (Overview, Groups, Roster,
Peers, Audit, Settings):
- Overview: version, node id, QUIC port, hub, index-cache maintenance
- Roster: node-wide view with unpin
- Peers and Audit: auto-load on open, no Load button
- Audit: real usernames and group names (resolved from the roster and
  node.toml), Previous/Next pagination newest-first, Export CSV of every
  matching row
- Settings: node settings, STUN, ICE, denylist, then Unlink from hub

Backend: audit.get_entries gains `offset`; /api/audit and /api/peers
resolve ids to names via a new _display_names helper; CSP tightened to
default-src 'none' now that no HTML is served. draft-v6 sections 2.11 and
2.12 corrected -- the Node page uses the loopback API, not MNP.

One capability is intentionally dropped: browser-based admin on a headless
server. The CLI covers every operation there.

See docs/refactor-node-ui.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MQCaZnde4Bjjdu84dhSuF5

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