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>

tests/test_selector.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from concurrent.futures import Future
 import curses
 from dataclasses import replace
 import os
@@ -1045,5 +1046,51 @@ class SelectorRobustnessTests(unittest.TestCase):
             self.assertIn("未登记", selector.message)
 
 
+class SelectorBackgroundLoadingTests(unittest.TestCase):
+    def test_local_sessions_arrive_after_the_popup_is_drawn(self) -> None:
+        with tempfile.TemporaryDirectory() as directory:
+            pending: Future[tuple[list[str], list[str]]] = Future()
+            selector = SessionSelector(
+                [],
+                RemoteRegistry(Path(directory) / "remotes.json"),
+                "/missing/ssh",
+                pending_local_sessions=pending,
+            )
+            self.addCleanup(selector.pool.close)
+            selector.message = "startup"
+
+            # Before the listing finishes the popup shows a placeholder row.
+            selector._poll_local_sessions()  # type: ignore[attr-defined]
+            kinds = {row.key: row for row in selector._rows()}  # type: ignore[attr-defined]
+            self.assertIn("loading:local", kinds)
+            self.assertFalse(kinds["loading:local"].selectable)
+            self.assertNotIn("local:work", kinds)
+
+            pending.set_result((["work", "default"], ["local Herdr session discovery was slow"]))
+            selector._poll_local_sessions()  # type: ignore[attr-defined]
+            self.assertIsNone(selector.pending_local_sessions)
+            keys = {row.key for row in selector._rows()}  # type: ignore[attr-defined]
+            self.assertIn("local:work", keys)
+            self.assertIn("local:default", keys)
+            self.assertNotIn("loading:local", keys)
+            self.assertEqual(selector.message, "startup; local Herdr session discovery was slow")
+
+    def test_failed_local_listing_becomes_a_message(self) -> None:
+        with tempfile.TemporaryDirectory() as directory:
+            pending: Future[tuple[list[str], list[str]]] = Future()
+            pending.set_exception(OSError("herdr is not executable"))
+            selector = SessionSelector(
+                [],
+                RemoteRegistry(Path(directory) / "remotes.json"),
+                "/missing/ssh",
+                pending_local_sessions=pending,
+            )
+            self.addCleanup(selector.pool.close)
+            selector._poll_local_sessions()  # type: ignore[attr-defined]
+            self.assertIsNone(selector.pending_local_sessions)
+            self.assertIn("herdr is not executable", selector.message)
+            self.assertEqual(selector.local_sessions, [])
+
+
 if __name__ == "__main__":
     unittest.main()