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/event_tree.json

@@ -0,0 +1,75 @@
+{
+  "tree": {
+    "min": 1,
+    "max": 4,
+    "size": 2,
+    "open_slots": 1,
+    "agents": [
+      {
+        "id": "lead",
+        "name": "lead",
+        "position": "the main agent",
+        "parent": null,
+        "state": "working",
+        "model": "qwen-27b",
+        "mode": "edit",
+        "request": [],
+        "manager": true,
+        "context": {
+          "used": 41200,
+          "window": 131072
+        },
+        "progress": {
+          "phase": "thinking",
+          "since": 1767323045000
+        },
+        "findings": 0,
+        "messages": 2
+      },
+      {
+        "id": "s2",
+        "name": "parser",
+        "position": "reviews the parser",
+        "parent": "lead",
+        "state": "working",
+        "model": "qwen-27b",
+        "mode": "manual",
+        "task": "Review src/parse.ts.",
+        "request": [
+          "write"
+        ],
+        "manager": false,
+        "context": {
+          "used": 9100,
+          "window": 131072
+        },
+        "progress": {
+          "phase": "running_tool",
+          "since": 1767323045000,
+          "tool": "read",
+          "what": "src/parse.ts"
+        },
+        "findings": 2,
+        "messages": 3
+      }
+    ],
+    "ended": [
+      {
+        "id": "s1",
+        "name": "scout",
+        "position": "maps the code",
+        "parent": "lead",
+        "state": "done",
+        "model": "qwen-27b",
+        "mode": "edit",
+        "task": "Map it.",
+        "request": [],
+        "manager": false,
+        "progress": null,
+        "findings": 1,
+        "messages": 1,
+        "ended": 1767323045000
+      }
+    ]
+  }
+}