feat(ui): add LandingPagePreview with Compose-based design preview

- Introduced `LandingPagePreview` composable for live UI previews in IntelliJ (Hot-Reload).
- Included fallback `main()` for standalone preview in the Design Preview Sandbox.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

frontend/shells/meldestelle-portal/src/jvmMain/kotlin/AppPreview.kt

@@ -0,0 +1,52 @@
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.Button
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.tooling.preview.Preview
+import androidx.compose.ui.window.Window
+import androidx.compose.ui.window.application
+import at.mocode.frontend.core.designsystem.theme.AppTheme
+import at.mocode.frontend.core.designsystem.theme.Dimens
+
+@Preview
+@Composable
+fun LandingPagePreview() {
+  AppTheme {
+    Surface(
+      modifier = Modifier.fillMaxSize(),
+      color = MaterialTheme.colorScheme.background
+    ) {
+      Column(
+        modifier = Modifier.padding(Dimens.SpacingM),
+        verticalArrangement = Arrangement.spacedBy(Dimens.SpacingM)
+      ) {
+        Text(
+          text = "Landing Page Preview",
+          style = MaterialTheme.typography.titleLarge,
+          color = MaterialTheme.colorScheme.primary
+        )
+        Text(
+          text = "Dies ist eine Live-Vorschau. Du kannst hier deine UI-Komponenten isoliert bauen und sofort sehen, wie sie aussehen. Ändere Text, Farben oder Layouts im Code – die Preview aktualisiert sich automatisch in IntelliJ (Hot-Reload).",
+          style = MaterialTheme.typography.bodyMedium,
+          color = MaterialTheme.colorScheme.onBackground
+        )
+        Button(onClick = {}) {
+          Text("Ein Button im Primary-Design")
+        }
+      }
+    }
+  }
+}
+
+// Fallback für IntelliJ, falls das Compose Plugin die Preview nicht erkennt
+fun main() = application {
+  Window(onCloseRequest = ::exitApplication, title = "Design Preview Sandbox") {
+    LandingPagePreview()
+  }
+}