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

src/Duempelkas.App/Components/Layout/MainLayout.razor

@@ -22,7 +22,7 @@
         <div class="dialog-content" @onclick:stopPropagation="true">
             <h5>Über Dümpelkas &middot; Version @AppVersion</h5>
             <p class="mb-2">Entwickler: Andre Beging</p>
-            <p class="mb-3">E-Mail: <a href="mailto:mail@beging.de" style="color: white;">mail@beging.de</a></p>
+            <p class="mb-3">E-Mail: <a href="mailto:mail@beging.de" style="color: var(--color-text);">mail@beging.de</a></p>
             <div class="d-flex justify-content-end">
                 <button type="button" class="btn btn-outline-secondary" @onclick="CloseAboutDialog">
                     <i class="bi bi-x-lg"></i> Schließen

src/Duempelkas.App/Pages/Dashboard.razor

@@ -39,6 +39,9 @@
         <button class="btn btn-nav btn-primary" @onclick="OpenYearFilterDialog">
             <i class="bi bi-funnel"></i> Ansicht<br>@GetFilterLabel()
         </button>
+        <button class="btn btn-nav btn-secondary" @onclick="ToggleThemeAsync" title="Farbmodus wechseln">
+            <i class="@ThemeButtonIconClass"></i> Modus<br>@ThemeButtonLabel
+        </button>
     </div>
 
     @if (accounts != null && accounts.Any())

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
 }