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_http.py

@@ -128,8 +128,8 @@ class TestServeDashboardFile(unittest.TestCase):
         handler._json_error = capture_error
 
         with tempfile.TemporaryDirectory() as tmpdir:
-            # Create a file outside the dashboard dir that shouldn't be accessible
-            secret = Path(tmpdir).parent / "secret.txt"
+            # Create a file outside the dashboard dir that shouldn't be accessible (unused - documents intent)
+            _secret = Path(tmpdir).parent / "secret.txt"
 
             with patch("amc_server.mixins.http.DASHBOARD_DIR", Path(tmpdir)):
                 handler._serve_dashboard_file("../secret.txt")