Address review findings on target_fps validation and config resolution

- Reject floats and bools before int() in the target_fps save path. A JSON
  body can carry them, where int(90.5) silently stored 90 and true stored 1.
  Form posts send strings, so '90.5' already failed in int().
- Assert the template's target_fps is 100, not merely an int, so the
  documented default is actually pinned.
- Empty-config precedence: keeping the `and config` check deliberately, now
  spelled out in the comment and covered by a test. Both managers default to
  the same config/config.json, so falling through cannot pick up a different
  file's settings; treating {} as an answer would instead return {} when the
  first manager simply hasn't loaded yet, silently disabling every setting
  read through the property -- the failure this property exists to fix.

Suite 1446 passed. The float-rejection test was checked to fail without the
guard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

test/test_web_api.py

@@ -201,6 +201,21 @@ class TestConfigAPI:
         saved = mock_config_manager.save_config_atomic.call_args[0][0]
         assert saved['web_display_autostart'] is True
 
+    @pytest.mark.parametrize('value', [90.5, 90.0, True])
+    def test_save_target_fps_rejects_non_integer_json(self, client, mock_config_manager, value):
+        """int() would truncate silently: 90.5 -> 90, True -> 1.
+
+        Only JSON can carry these; a form post sends '90.5', which int()
+        already rejects.
+        """
+        response = client.post(
+            '/api/v3/config/main',
+            data=json.dumps({'target_fps': value}),
+            content_type='application/json',
+        )
+
+        assert response.status_code == 400
+
     @pytest.mark.parametrize('value', ['20', '250', 'fast'])
     def test_save_target_fps_rejects_out_of_range(self, client, mock_config_manager, value):
         """Values ScrollHelper would silently clamp are reported instead."""