feat: add loadGridViewMode and saveGridViewMode with local/server scope

- Add loadGridViewMode() that reads grid view mode preference based on
  viewPreferenceScope in display settings: returns gridViewMode from
  localStorage display settings when scope is 'local' (default), or
  grid_view_mode from _serverSettings when scope is 'server'. Returns
  'flat' as default when no preference is set.
- Add saveGridViewMode(mode) that saves to appropriate scope (localStorage
  via saveDisplaySettings when local, patchServerSetting when server)
  and updates _gridViewMode module variable.
- Wire loadGridViewMode() into DOMContentLoaded startup after applyDisplaySettings
  so _gridViewMode is initialized from stored preference on page load.
- Export both functions from module.exports for testing.

Closes task-13

muxplex/frontend/app.js

@@ -1233,6 +1233,40 @@ function applyDisplaySettings(ds) {
   }
 }
 
+/**
+ * Load grid view mode preference based on viewPreferenceScope in display settings.
+ * When scope is 'local' (default), reads gridViewMode from display settings (localStorage).
+ * When scope is 'server', reads grid_view_mode from _serverSettings.
+ * Returns 'flat' as default.
+ * @returns {string}
+ */
+function loadGridViewMode() {
+  var ds = loadDisplaySettings();
+  var scope = ds.viewPreferenceScope || 'local';
+  if (scope === 'server') {
+    return (_serverSettings && _serverSettings.grid_view_mode) || 'flat';
+  }
+  return ds.gridViewMode || 'flat';
+}
+
+/**
+ * Save grid view mode preference to the appropriate scope and update _gridViewMode.
+ * When scope is 'local', saves gridViewMode to display settings (localStorage).
+ * When scope is 'server', patches the server setting via patchServerSetting.
+ * @param {string} mode - The grid view mode to save.
+ */
+function saveGridViewMode(mode) {
+  var ds = loadDisplaySettings();
+  var scope = ds.viewPreferenceScope || 'local';
+  if (scope === 'server') {
+    patchServerSetting('grid_view_mode', mode);
+  } else {
+    ds.gridViewMode = mode;
+    saveDisplaySettings(ds);
+  }
+  _gridViewMode = mode;
+}
+
 /**
  * Handle a change event on any Display settings control.
  * Reads current values from form elements, saves via saveDisplaySettings,
@@ -1913,6 +1947,7 @@ function _setActiveFilterDevice(device) {
 document.addEventListener('DOMContentLoaded', () => {
   initDeviceId();
   applyDisplaySettings(loadDisplaySettings());
+  _gridViewMode = loadGridViewMode();
   document.addEventListener('keydown', trackInteraction);
   document.addEventListener('click', trackInteraction);
   document.addEventListener('touchstart', trackInteraction);
@@ -1981,6 +2016,8 @@ if (typeof module !== 'undefined' && module.exports) {
     loadDisplaySettings,
     saveDisplaySettings,
     applyDisplaySettings,
+    loadGridViewMode,
+    saveGridViewMode,
     onDisplaySettingChange,
     openSettings,
     closeSettings,

muxplex/frontend/tests/test_app.mjs

@@ -2608,4 +2608,62 @@ test('renderFilterBar and _setActiveFilterDevice are exported', () => {
   assert.strictEqual(typeof app._setActiveFilterDevice, 'function', '_setActiveFilterDevice should be exported');
 });
 
+// --- loadGridViewMode / saveGridViewMode (task-13) ---
+
+test('loadGridViewMode returns flat by default', () => {
+  // Clear display settings and server settings so everything is at defaults
+  _localStorageStore = {};
+  app._setServerSettings(null);
+
+  const mode = app.loadGridViewMode();
+  assert.strictEqual(mode, 'flat', 'loadGridViewMode should return flat when no preference is set');
+});
+
+test('loadGridViewMode reads from localStorage when scope is local', () => {
+  // Set display settings with viewPreferenceScope 'local' and gridViewMode 'grouped'
+  _localStorageStore = {};
+  _localStorageStore['muxplex.display'] = JSON.stringify({ viewPreferenceScope: 'local', gridViewMode: 'grouped' });
+  app._setServerSettings(null);
+
+  const mode = app.loadGridViewMode();
+  assert.strictEqual(mode, 'grouped', 'loadGridViewMode should return gridViewMode from localStorage when scope is local');
+
+  // Cleanup
+  _localStorageStore = {};
+});
+
+test('loadGridViewMode reads from serverSettings when scope is server', () => {
+  // Set display settings with viewPreferenceScope 'server', server has grid_view_mode 'filtered'
+  _localStorageStore = {};
+  _localStorageStore['muxplex.display'] = JSON.stringify({ viewPreferenceScope: 'server' });
+  app._setServerSettings({ grid_view_mode: 'filtered' });
+
+  const mode = app.loadGridViewMode();
+  assert.strictEqual(mode, 'filtered', 'loadGridViewMode should return grid_view_mode from serverSettings when scope is server');
+
+  // Cleanup
+  _localStorageStore = {};
+  app._setServerSettings(null);
+});
+
+test('saveGridViewMode stores to localStorage when scope is local', () => {
+  // Set scope to local (default)
+  _localStorageStore = {};
+  _localStorageStore['muxplex.display'] = JSON.stringify({ viewPreferenceScope: 'local' });
+  app._setServerSettings(null);
+
+  app.saveGridViewMode('grouped');
+
+  // Verify _gridViewMode was updated
+  assert.strictEqual(app._getGridViewMode(), 'grouped', '_gridViewMode should be set to grouped');
+
+  // Verify it was saved to localStorage display settings
+  const saved = JSON.parse(_localStorageStore['muxplex.display'] || '{}');
+  assert.strictEqual(saved.gridViewMode, 'grouped', 'gridViewMode should be saved to localStorage display settings');
+
+  // Cleanup
+  _localStorageStore = {};
+  app._setGridViewMode('flat');
+});
+