Persist frontend theme preference

src/RolemasterDb.App/Frontend/AppState/BrowserStorageService.cs

@@ -0,0 +1,12 @@
+using Microsoft.JSInterop;
+
+namespace RolemasterDb.App.Frontend.AppState;
+
+public sealed class BrowserStorageService(IJSRuntime jsRuntime)
+{
+    public ValueTask<string?> GetItemAsync(string key) =>
+        jsRuntime.InvokeAsync<string?>("localStorage.getItem", key);
+
+    public ValueTask SetItemAsync(string key, string value) =>
+        jsRuntime.InvokeVoidAsync("localStorage.setItem", key, value);
+}