fix(web): treat only 2xx as a successful display save

Review feedback on #422.

- showDisplaySaveResult tested `xhr.status >= 400` for failure, so a
  network error — which reports status 0 — was waved through as
  "Display settings saved". That's the same class of false-success bug
  this branch set out to fix. Test the 2xx range instead, and let a
  response body refine a successful verdict without overturning a
  failed one.
- Annotate the _copies_fits_hardware helper, matching the annotated
  helpers already in api_v3.py.
- Cover the vertical divisibility branch: chain_length 2 would divide
  evenly, so only parallel 3 can produce the rejection, which pins the
  branch to the right hardware dimension.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016aUvzTXpEYhNEWYQvFqQU9

test/test_web_api.py

@@ -225,6 +225,27 @@ class TestConfigAPI:
         assert 'chain length' in response.get_json()['message']
         mock_config_manager.save_config_atomic.assert_not_called()
 
+    def test_save_double_sided_vertical_checks_parallel(self, client, mock_config_manager):
+        """The vertical axis is checked against parallel, not chain_length."""
+        mock_config_manager.load_config.return_value['display']['hardware'] = {
+            'chain_length': 2, 'parallel': 3,
+        }
+
+        response = client.post(
+            '/api/v3/config/main',
+            data={
+                'double_sided_enabled': 'true',
+                'double_sided_copies': '2',
+                'double_sided_axis': 'vertical',
+            },
+            content_type='application/x-www-form-urlencoded',
+        )
+
+        # chain_length 2 would divide evenly — only parallel 3 rejects this.
+        assert response.status_code == 400
+        assert 'parallel' in response.get_json()['message']
+        mock_config_manager.save_config_atomic.assert_not_called()
+
     def test_save_double_sided_disabled_ignores_bad_values(self, client, mock_config_manager):
         """While disabled, unusable copies/axis are dropped rather than rejected."""
         mock_config_manager.load_config.return_value['display']['double_sided'] = {