Add Blazor application layer with UI components and pages

- Service interfaces and DTO models
- Dashboard page with account overview
- Account detail page with year/entry management
- Reusable components: AccountCard, EntryTable, YearSelector
- Dialog components: Add/Edit Account, Entry, Transfer, Year
- Main layout and routing configuration

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

@@ -0,0 +1,31 @@
+@inherits LayoutComponentBase
+
+<div class="d-flex flex-column vh-100">
+    <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>
+        <span class="version-label">v@(AppVersion)</span>
+    </nav>
+
+    <main class="flex-grow-1 overflow-auto p-3">
+        @Body
+    </main>
+
+    <footer class="app-footer">
+        Dümpelkas &copy; @DateTime.Now.Year &middot; Version @AppVersion
+    </footer>
+</div>
+
+@code {
+    private static string AppVersion
+    {
+        get
+        {
+            var now = DateTime.Now;
+            var yearShort = now.Year % 100;
+            var dayOfYear = now.DayOfYear;
+            return $"{yearShort}.{dayOfYear}";
+        }
+    }
+}