Load local sessions in the background for the Sessions popup

command_choose ran `herdr session list --json` (up to its 5 s timeout)
before the popup drew anything, contradicting the documented immediate
display. Run the listing on a worker thread and hand the selector a
Future: the popup appears at once with a "… 本机 正在查询" placeholder,
then fills in the local rows and any discovery warning when the result
arrives, mirroring how remote hosts already load.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

herdr_hub/cli.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import argparse
+from concurrent.futures import ThreadPoolExecutor
 from contextlib import contextmanager
 import curses
 from dataclasses import dataclass, field, replace
@@ -1389,13 +1390,19 @@ def command_open(arguments: argparse.Namespace) -> int:
     return 0
 
 
+def local_session_listing(herdr_bin: str) -> tuple[list[str], list[str]]:
+    """Background half of the Sessions popup: names and warnings only."""
+    report = discover_local_sessions(herdr_bin)
+    return [endpoint.session for endpoint in report.endpoints], report.warnings
+
+
 def command_choose(arguments: argparse.Namespace) -> int:
     startup_warnings: list[str] = []
     hub = make_hub(arguments, warnings=startup_warnings)
     if not hub.exists():
         raise HubError("herdr-hub is not running")
     windows = hub.managed_windows()
-    local_report = discover_local_sessions(resolve_herdr())
+    herdr_bin = resolve_herdr()
     registry_warning = remember_remote_endpoints(
         window.endpoint for window in windows if window.endpoint is not None
     )
@@ -1403,6 +1410,11 @@ def command_choose(arguments: argparse.Namespace) -> int:
     alias_registry = make_alias_registry()
     remote_registry = make_registry()
     ssh_bin = resolve_ssh()
+    # `herdr session list` can take seconds; the popup must not wait for it.
+    local_executor = ThreadPoolExecutor(
+        max_workers=1, thread_name_prefix="herdr-hub-local"
+    )
+    local_future = local_executor.submit(local_session_listing, herdr_bin)
 
     def close_window(window_id: str) -> list[CatalogWindow]:
         hub.dismiss_window(window_id)
@@ -1519,15 +1531,14 @@ def command_choose(arguments: argparse.Namespace) -> int:
             catalog_windows(windows),
             remote_registry,
             ssh_bin,
-            local_sessions=(endpoint.session for endpoint in local_report.endpoints),
             aliases=hub.aliases,
             activate_handler=activate_selection,
             rename_handler=rename_session,
             close_window_handler=close_window,
             delete_session_handler=delete_session,
             stop_session_handler=stop_session,
+            pending_local_sessions=local_future,
         )
-        startup_warnings.extend(local_report.warnings)
         if registry_warning:
             startup_warnings.append(registry_warning)
         if startup_warnings:
@@ -1535,6 +1546,8 @@ def command_choose(arguments: argparse.Namespace) -> int:
         result = selector.run()
     except (RegistryError, SelectorError) as error:
         raise HubError(str(error)) from error
+    finally:
+        local_executor.shutdown(wait=False, cancel_futures=True)
     if result is None:
         return 0
     if result.kind == "rename":