feat(linux): add graphical platform services

src/AcDream.UI.Abstractions/Panels/Settings/SettingsStore.cs

@@ -13,7 +13,8 @@ public readonly record struct UiWindowPosition(float X, float Y);
 /// <summary>
 /// JSON-backed persistence for non-keybind settings (Display today; future
 /// tabs Audio / Gameplay / Chat / Character will be added to the same
-/// file). Path: <c>%LOCALAPPDATA%\acdream\settings.json</c>. Coexists
+/// file). The graphical host supplies a canonical portable configuration
+/// path. Coexists
 /// with <c>keybinds.json</c>, which retains its own
 /// <see cref="Input.KeyBindings.LoadOrDefault"/> path.
 ///
@@ -41,12 +42,6 @@ public sealed class SettingsStore
         _path = path ?? throw new ArgumentNullException(nameof(path));
     }
 
-    /// <summary>Default path: <c>%LOCALAPPDATA%\acdream\settings.json</c>.</summary>
-    public static string DefaultPath() => Path.Combine(
-        Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
-        "acdream",
-        "settings.json");
-
     /// <summary>
     /// Load Display settings. Missing file → <see cref="DisplaySettings.Default"/>.
     /// Missing individual keys fall back to the corresponding default