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_spawn.py

@@ -7,7 +7,7 @@ from pathlib import Path
 from unittest.mock import MagicMock, patch
 
 import amc_server.mixins.spawn as spawn_mod
-from amc_server.mixins.spawn import SpawnMixin, load_projects_cache, _projects_cache, _sanitize_pane_name
+from amc_server.mixins.spawn import SpawnMixin, load_projects_cache, _sanitize_pane_name
 
 
 class DummySpawnHandler(SpawnMixin):
@@ -317,7 +317,7 @@ class TestRateLimiting(unittest.TestCase):
 
     def test_rapid_spawn_same_project_rejected(self):
         """Spawning the same project within cooldown returns 429."""
-        from amc_server.context import _spawn_timestamps, SPAWN_COOLDOWN_SEC
+        from amc_server.context import _spawn_timestamps
         _spawn_timestamps.clear()
         # Pretend we just spawned this project
         _spawn_timestamps['rapid-project'] = time.monotonic()