Add Host Agent local console task cancellation (task-cancellation 7.1-7.3)

- New internal API route POST /internal/v1/hosts/{host_id}/tasks/{task_id}/cancel,
  authenticated via the host's own bearer credential (authorize_host) with an
  ownership check, since host tokens carry no scopes and cannot reach the
  public SDK's tasks:submit-scoped cancel endpoint.
- HostAgentClient.cancel_task() calls the new internal route directly.
- create_console_app() gains a cancel_task callable with automatic default
  wiring from host_client, so production app.py needs no changes.
- Local console: POST /tasks/{task_id}/cancel route resolves the local
  execution id to its Cloud source_task_id before cancelling, and the task
  detail page/template show a Cancel button plus notice/error banners.
- Tests across all three layers: internal API route, Jinja2 template
  rendering, and FastAPI console route behavior.

apps/device-host-agent/tests/host_agent/web/conftest.py

@@ -193,6 +193,9 @@ def make_task_detail_context(
     task: dict[str, Any] | None = None,
     task_rows: list[tuple[str, Any]] | None = None,
     timeline_steps: list[dict[str, Any]] | None = None,
+    can_cancel: bool = False,
+    cancel_notice: str | None = None,
+    cancel_error: str | None = None,
 ) -> dict[str, Any]:
     if task is None:
         task = {
@@ -227,7 +230,11 @@ def make_task_detail_context(
     return {
         "title": "Task task-001",
         "session": session,
+        "csrf_token": session.csrf_token,
         "task": task,
         "task_rows": task_rows,
         "timeline_steps": timeline_steps,
+        "can_cancel": can_cancel,
+        "cancel_notice": cancel_notice,
+        "cancel_error": cancel_error,
     }