Restore Settings as sidebar item on macOS

Remove the gear toolbar button that opened Settings as a sheet in the
NavigationSplitView sidebar column, and drop the macOS guard hiding
.settings from SidebarMainItem so it can be added to the sidebar and
rendered in the detail column like other items. The dedicated Settings
window (Cmd+,) is unchanged.

Yattee/Models/Navigation/SidebarMainItem.swift

@@ -124,12 +124,6 @@ enum SidebarMainItem: String, CaseIterable, Codable, Identifiable, Sendable {
             #else
             return true
             #endif
-        case .settings:
-            #if os(macOS)
-            return false
-            #else
-            return true
-            #endif
         default:
             return true
         }