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_state.py

@@ -4,7 +4,7 @@ import tempfile
 import time
 import unittest
 from pathlib import Path
-from unittest.mock import patch, MagicMock
+from unittest.mock import patch
 
 import amc_server.mixins.state as state_mod
 from amc_server.mixins.state import StateMixin