feat(readability): add scalable text and responsive high-contrast layouts (U13)

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

@@ -1,12 +1,20 @@
 @inherits LayoutComponentBase
+@inject IJSRuntime JS
 
 <ToastHost />
 
-<div class="d-flex flex-column vh-100">
+<div class="app-shell">
     <nav class="app-navbar d-flex justify-content-between align-items-center">
         <a class="navbar-brand" href="">
             <strong>Dümpelkas</strong> <small class="text-muted">Kassenbuch</small>
         </a>
+        <div class="reading-controls">
+            <label for="text-scale">Textgröße</label>
+            <select id="text-scale" @bind="fontScale" @bind:after="ApplyTextScale">
+                <option value="100">Normal</option><option value="125">Groß (125 %)</option>
+                <option value="150">Sehr groß (150 %)</option><option value="200">200 %</option>
+            </select>
+        </div>
     </nav>
 
     <main class="flex-grow-1 overflow-auto p-3">