refactor(ui, navigation): implement platform-specific routing and redesign components

- Added platform detection logic `currentPlatform()` in `PlatformType.js.kt`.
- Introduced platform-based behavior for LandingScreen, Dashboard, and Login flow.
- Replaced Row with FlowRow in PingScreen to improve button layout.
- Updated Meldestelle Dashboard with platform-specific headers and authentication checks.
- Adjusted AppHeader to accept `isAuthenticated` and `username` parameters.

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

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/components/AppHeader.kt

@@ -1,74 +1,67 @@
 package at.mocode.frontend.core.designsystem.components
 
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
 import androidx.compose.material3.*
 import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
 import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
 
-@OptIn(ExperimentalMaterial3Api::class)
 @Composable
 fun AppHeader(
-  title: String,
-  onNavigateToPing: (() -> Unit)? = null,
+  isAuthenticated: Boolean,
+  username: String?,
   onNavigateToLogin: (() -> Unit)? = null,
-  onLogout: (() -> Unit)? = null,
-  isAuthenticated: Boolean = false,
-  username: String? = null,
-  userPermissions: List<String> = emptyList()
+  onLogout: (() -> Unit)? = null
 ) {
-  TopAppBar(
-    title = {
+  Surface(
+    color = MaterialTheme.colorScheme.surface,
+    shadowElevation = 2.dp,
+    modifier = Modifier.fillMaxWidth()
+  ) {
+    Row(
+      modifier = Modifier.padding(horizontal = 24.dp, vertical = 16.dp),
+      verticalAlignment = Alignment.CenterVertically,
+      horizontalArrangement = Arrangement.SpaceBetween
+    ) {
       Text(
-        text = title,
-        style = MaterialTheme.typography.headlineSmall,
-        fontWeight = FontWeight.Bold
+        text = "Meldestelle",
+        style = MaterialTheme.typography.titleLarge,
+        fontWeight = FontWeight.Bold,
+        color = MaterialTheme.colorScheme.primary
       )
-    },
-    actions = {
-      // Ping Service button
-      onNavigateToPing?.let { navigateAction ->
-        TextButton(
-          onClick = navigateAction
-        ) {
-          Text("Ping Service")
-        }
-      }
 
-      // Authentication buttons
-      if (isAuthenticated) {
-        // Show username with admin indicator if user has deleted permissions
-        username?.let { user ->
-          val isAdmin = userPermissions.any { it.contains("DELETE") }
+      Row(
+        verticalAlignment = Alignment.CenterVertically,
+        horizontalArrangement = Arrangement.spacedBy(16.dp)
+      ) {
+        if (isAuthenticated) {
+          // Show the username and logout button
           Text(
-            text = if (isAdmin) "👑 Hallo, $user (Admin)" else "Hallo, $user",
+            text = "Angemeldet als: ${username ?: "Admin"}",
             style = MaterialTheme.typography.bodyMedium,
-            color = if (isAdmin)
-              MaterialTheme.colorScheme.tertiary
-            else
-              MaterialTheme.colorScheme.onPrimaryContainer
+            color = MaterialTheme.colorScheme.onSurfaceVariant
           )
-        }
-        onLogout?.let { logoutAction ->
-          TextButton(
-            onClick = logoutAction
-          ) {
-            Text("Abmelden")
+          onLogout?.let { logoutAction ->
+            TextButton(onClick = logoutAction) {
+              Text("Abmelden")
+            }
           }
-        }
-      } else {
-        // Show the login button
-        onNavigateToLogin?.let { loginAction ->
-          TextButton(
-            onClick = loginAction
-          ) {
-            Text("Anmelden")
+        } else {
+          // Show the login button
+          onNavigateToLogin?.let { loginAction ->
+            Button(
+              onClick = loginAction
+            ) {
+              Text("Login")
+            }
           }
         }
       }
-    },
-    colors = TopAppBarDefaults.topAppBarColors(
-      containerColor = MaterialTheme.colorScheme.primaryContainer,
-      titleContentColor = MaterialTheme.colorScheme.onPrimaryContainer,
-      actionIconContentColor = MaterialTheme.colorScheme.onPrimaryContainer
-    )
-  )
+    }
+  }
 }

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/components/AppScaffold.kt

@@ -12,7 +12,7 @@ import androidx.compose.ui.Modifier
 @Composable
 fun AppScaffold(
   header: @Composable () -> Unit = {
-    AppHeader(title = "Meldestelle")
+    AppHeader(isAuthenticated = false, username = null)
   },
   content: @Composable (PaddingValues) -> Unit,
   footer: @Composable () -> Unit = {