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_hook.py

@@ -6,7 +6,6 @@ Edge cases are prioritized over happy paths.
 
 import json
 import os
-import sys
 import tempfile
 import types
 import unittest
@@ -193,7 +192,7 @@ class TestAtomicWrite(unittest.TestCase):
             path.write_text('{"original": "data"}')
 
             # Mock os.replace to fail after the temp file is written
-            original_replace = os.replace
+            _original_replace = os.replace  # noqa: F841 - documents test setup
             def failing_replace(src, dst):
                 raise PermissionError("Simulated failure")