refactor(dashboard): change SpawnModal from overlay modal to dropdown

Position the spawn modal directly under the 'New Agent' button without a
blur overlay. Uses click-outside dismissal and absolute positioning.
Reduces visual disruption for quick agent spawning.

tests/test_control.py

@@ -325,8 +325,8 @@ class TestDismissSession(unittest.TestCase):
         with tempfile.TemporaryDirectory() as tmpdir:
             sessions_dir = Path(tmpdir)
             sessions_dir.mkdir(exist_ok=True)
-            # Create a file that should NOT be deleted
-            secret_file = Path(tmpdir).parent / "secret.json"
+            # Create a file that should NOT be deleted (unused - documents test intent)
+            _secret_file = Path(tmpdir).parent / "secret.json"
 
             handler = DummyControlHandler()
             with patch.object(control, "SESSIONS_DIR", sessions_dir):