fix(web): don't validate double-sided settings when the feature is disabled

Saving anything on the Display tab failed with a 400 when double-sided
mode was off:

    Double-sided copies (2) must divide chain length (3) evenly

The Display form posts every field in one request, including
double_sided_copies (default 2) and double_sided_axis, whether or not
the Enabled checkbox is ticked. The server block was gated only on "is
any double-sided field present in the payload" — it wrote
ds_config['enabled'] but never read it. A user with chain_length: 3 and
the untouched default copies: 2 was locked out of saving any display
setting at all: brightness, GPIO slowdown, Vegas, sync.

Gate the checks on the enabled flag:

- Divisibility against chain_length/parallel is hardware-relational and
  only runs when the feature is on.
- Structural checks (copies parses as an int in 2..8, axis in the
  whitelist) still 400 when enabled; when disabled they drop the value
  and leave the stored one untouched rather than rejecting the save.

The runtime already gated correctly (_resolve_double_sided returns None
when disabled), so nothing there changes.

Also in the Display tab:

- Hide Copies / Split Axis until Enabled is ticked, mirroring the Vegas
  Scroll pattern. Hidden rather than disabled, so the fields keep
  submitting and the server still sees an 'off' state to persist.
- Fix the save toast: the form's handler read xhr.responseJSON, a jQuery
  property that doesn't exist on a native XMLHttpRequest, so it was
  always undefined and every save reported a green "Display settings
  saved" — even the 400s. Parse responseText and use the real status.

Tests: two existing double-sided tests asserted 200 on payloads that the
divisibility check (added later, in #373) turns into 400s; the first now
supplies matching hardware values and the second passes as written now
that a disabled save skips the check. Added coverage for the reported
regression, for bad values while disabled, and for the check still
firing when enabled.

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

test/test_web_api.py

@@ -146,6 +146,11 @@ class TestConfigAPI:
 
     def test_save_double_sided_settings(self, client, mock_config_manager):
         """Double-sided form fields are persisted under display.double_sided."""
+        # 2 copies on the vertical axis needs parallel to be a multiple of 2.
+        mock_config_manager.load_config.return_value['display']['hardware'] = {
+            'chain_length': 2, 'parallel': 2,
+        }
+
         response = client.post(
             '/api/v3/config/main',
             data={
@@ -175,6 +180,70 @@ class TestConfigAPI:
         assert ds['enabled'] is False
         assert ds['copies'] == 4
 
+    def test_save_double_sided_disabled_skips_divisibility_check(self, client, mock_config_manager):
+        """A copies/chain_length mismatch must not block saves while disabled.
+
+        The Display form posts copies/axis on every save, so validating them
+        with the feature off locked users out of every other display setting.
+        """
+        mock_config_manager.load_config.return_value['display']['hardware'] = {
+            'chain_length': 3, 'parallel': 1,
+        }
+
+        response = client.post(
+            '/api/v3/config/main',
+            data={
+                'double_sided_copies': '2',
+                'double_sided_axis': 'horizontal',
+                'brightness': '75',
+            },
+            content_type='application/x-www-form-urlencoded',
+        )
+
+        assert response.status_code == 200
+        ds = mock_config_manager.save_config_atomic.call_args[0][0]['display']['double_sided']
+        assert ds['enabled'] is False
+        assert ds['copies'] == 2
+
+    def test_save_double_sided_enabled_enforces_divisibility(self, client, mock_config_manager):
+        """The same mismatch is still rejected once the feature is turned on."""
+        mock_config_manager.load_config.return_value['display']['hardware'] = {
+            'chain_length': 3, 'parallel': 1,
+        }
+
+        response = client.post(
+            '/api/v3/config/main',
+            data={
+                'double_sided_enabled': 'true',
+                'double_sided_copies': '2',
+                'double_sided_axis': 'horizontal',
+            },
+            content_type='application/x-www-form-urlencoded',
+        )
+
+        assert response.status_code == 400
+        assert 'chain length' 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'] = {
+            'enabled': True, 'copies': 2, 'axis': 'horizontal',
+        }
+
+        response = client.post(
+            '/api/v3/config/main',
+            data={'double_sided_copies': 'abc', 'double_sided_axis': 'diagonal'},
+            content_type='application/x-www-form-urlencoded',
+        )
+
+        assert response.status_code == 200
+        ds = mock_config_manager.save_config_atomic.call_args[0][0]['display']['double_sided']
+        assert ds['enabled'] is False
+        # Stored values left untouched rather than overwritten with junk.
+        assert ds['copies'] == 2
+        assert ds['axis'] == 'horizontal'
+
     def test_save_double_sided_invalid_copies_rejected(self, client, mock_config_manager):
         """copies < 2 is rejected with a 400 before any save."""
         response = client.post(