Remove "Link all monitor brightness" feature

Eliminates the UI and backend logic for synchronizing all monitor brightness levels. Refactors monitor list update logic to distinguish between initial load and refresh, introducing RestoreMonitorSettings for startup state restoration. Streamlines feature visibility application and settings handling for improved maintainability.

src/modules/powerdisplay/PowerDisplay/Helpers/MonitorManager.cs

@@ -182,28 +182,6 @@ namespace PowerDisplay.Helpers
                 (mon, val) => mon.UpdateStatus(val, true),
                 cancellationToken);
 
-        /// <summary>
-        /// Set brightness of all monitors
-        /// </summary>
-        public async Task<IEnumerable<MonitorOperationResult>> SetAllBrightnessAsync(int brightness, CancellationToken cancellationToken = default)
-        {
-            var tasks = _monitors
-                .Where(m => m.IsAvailable)
-                .Select(async monitor =>
-                {
-                    try
-                    {
-                        return await SetBrightnessAsync(monitor.Id, brightness, cancellationToken);
-                    }
-                    catch (Exception ex)
-                    {
-                        return MonitorOperationResult.Failure($"Failed to set brightness for {monitor.Name}: {ex.Message}");
-                    }
-                });
-
-            return await Task.WhenAll(tasks);
-        }
-
         /// <summary>
         /// Set contrast of the specified monitor
         /// </summary>