refactor(app): add theme toggle functionality and improve theme management

src/Duempelkas.App/Pages/Dashboard.razor.cs

@@ -1,5 +1,7 @@
 using System.Globalization;
 using Duempelkas.App.Services.Models;
+using Microsoft.AspNetCore.Components;
+using Microsoft.JSInterop;
 
 namespace Duempelkas.App.Pages;
 
@@ -18,6 +20,7 @@ public partial class Dashboard
     private string? operationMessage;
     private string operationMessageClass = "alert-info";
     private string? savedPdfPath;
+    private string currentTheme = "light";
 
     #endregion
 
@@ -37,6 +40,26 @@ public partial class Dashboard
         await LoadAll();
     }
 
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (!firstRender)
+        {
+            return;
+        }
+
+        try
+        {
+            currentTheme = await JS.InvokeAsync<string>("duempelkasThemeGetCurrentTheme");
+        }
+        catch (JSException)
+        {
+            // Keep default light mode when JS interop is not ready or unavailable.
+            currentTheme = "light";
+        }
+
+        StateHasChanged();
+    }
+
     #endregion
 
     #region Data Loading
@@ -167,5 +190,24 @@ public partial class Dashboard
 
     private string GetFilterLabel() => selectedYear?.ToString() ?? "Alle";
 
+    private string ThemeButtonLabel => currentTheme == "dark" ? "Hell" : "Dunkel";
+
+    private string ThemeButtonIconClass => currentTheme == "dark" ? "bi bi-sun" : "bi bi-moon-stars";
+
+    private async Task ToggleThemeAsync()
+    {
+        try
+        {
+            currentTheme = await JS.InvokeAsync<string>("duempelkasThemeToggleTheme");
+        }
+        catch (JSException)
+        {
+            currentTheme = currentTheme == "dark" ? "light" : "dark";
+        }
+    }
+
+    [Inject]
+    private IJSRuntime JS { get; set; } = default!;
+
     #endregion
 }