Subagents (C4): the tree in the activity panel, read-only sessions, findings, stop; agent chats' setting

On the server's /api/v2 (3.4.0, server dev 5d7b79c):
- The activity panel reads GET /chats/{chat}/tree beside the activity and
  draws ui/js/tree.js's rows in a Subagents section, the server panel's words
  and layout: nested by parent, position, phase, context %, state, model,
  mode + request options, messages and the last one, open slots, the ended
  ones folded and dimmed. A team's members are no longer drawn again among
  the tasks. The stream's `tree` frame redraws it.
- A subagent's conversation (/subagents/{id}/transcript, read again every 3 s
  while it works) and its findings open read-only in the panel; Stop asks,
  then POSTs /stop. With an agent chat's device away: the kept tree with the
  server's note, and its 409 sentence for the rest.
- A new agent chat's Subagents chip from /models?kind=agent&device= (each
  model's `subagents`, `agent_subagents.offered` and `device_runs`); an open
  agent chat's from its own `subagents`, as before.
- An approval item's `subagent` names it on the card.
- Progress keys are the spec's own ({{field}}), as the server's catalogues
  have them; SERVER_PENDING is gone, every sentence is translated.

tests/ui reads the server's contract fixtures (scripts/fetch-fixtures.sh ->
tests/ui/fixtures, SOURCE says the commit): status frames, tree, tree frame,
transcript, findings, models?kind=agent, a subagent's approval; six C4 cases.

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

tests/ui/fixtures/chat_agent.json

@@ -0,0 +1,31 @@
+{
+  "id": "chat2",
+  "kind": "agent",
+  "title": "Waybread",
+  "model": "qwen",
+  "folder_id": null,
+  "pinned": false,
+  "archived": false,
+  "temporary": false,
+  "unread": true,
+  "running": true,
+  "mode": "edit",
+  "effort": "low",
+  "device": {
+    "token_id": "token1",
+    "directory": "/home/dev/project"
+  },
+  "device_session_id": "ses_01waybread",
+  "parent_chat_id": null,
+  "agents": {
+    "min": 1,
+    "max": 4
+  },
+  "subagents": {
+    "offered": true,
+    "named_only": false,
+    "cap": 32
+  },
+  "created_at": "2026-01-02T03:04:05+00:00",
+  "updated_at": "2026-01-02T03:04:05+00:00"
+}