chore: remove obsolete screens from meldestelle-desktop module

- Deleted unused screens including `AdminUebersichtScreen`, `AktorScreens`, `StammdatenImportScreen`, `TurnierDetailScreen`, and supporting components such as `PlaceholderContent`.
- Cleaned up references and placeholders to streamline module structure.

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

frontend/core/design-system/build.gradle.kts

@@ -21,6 +21,7 @@ kotlin {
       implementation(compose.runtime)
       implementation(compose.foundation)
       implementation(compose.material3)
+      implementation(compose.materialIconsExtended)
       implementation(compose.ui)
       implementation(compose.components.resources)
       implementation(libs.bundles.kmp.common)

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/models/PlaceholderContent.kt

@@ -0,0 +1,47 @@
+package at.mocode.frontend.core.designsystem.models
+
+import androidx.compose.foundation.layout.*
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.Construction
+import androidx.compose.material3.Icon
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+
+/**
+ * Wiederverwendbarer Platzhalter für Screens, die noch nicht implementiert sind.
+ */
+@Composable
+fun PlaceholderContent(
+  title: String,
+  subtitle: String = "Wird in einer späteren Phase implementiert.",
+) {
+  Box(
+    modifier = Modifier.fillMaxSize(),
+    contentAlignment = Alignment.Center,
+  ) {
+    Column(horizontalAlignment = Alignment.CenterHorizontally) {
+      Icon(
+        imageVector = Icons.Default.Construction,
+        contentDescription = null,
+        modifier = Modifier.size(64.dp),
+        tint = MaterialTheme.colorScheme.outlineVariant,
+      )
+      Spacer(modifier = Modifier.height(16.dp))
+      Text(
+        text = title,
+        style = MaterialTheme.typography.titleMedium,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+      )
+      Spacer(modifier = Modifier.height(8.dp))
+      Text(
+        text = subtitle,
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.outline,
+      )
+    }
+  }
+}

frontend/core/design-system/src/commonMain/kotlin/at/mocode/frontend/core/designsystem/models/SharedUiModels.kt

@@ -0,0 +1,38 @@
+package at.mocode.frontend.core.designsystem.models
+
+import androidx.compose.foundation.layout.padding
+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.graphics.Color
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+
+// ── Gemeinsame Enums ─────────────────────────────────────────────────────────
+enum class LoginStatus { AKTIV, AUSSTEHEND }
+enum class VeranstaltungStatus { VORBEREITUNG, LIVE, ABGESCHLOSSEN }
+
+// ── Gemeinsame Composables ───────────────────────────────────────────────────
+@Composable
+fun LoginStatusBadge(status: LoginStatus, modifier: Modifier = Modifier) {
+  val (text, color) = when (status) {
+    LoginStatus.AKTIV -> "Aktiv" to Color(0xFF16A34A)
+    LoginStatus.AUSSTEHEND -> "Ausstehend" to Color(0xFFEA580C)
+  }
+  Surface(
+    shape = MaterialTheme.shapes.extraSmall,
+    color = color.copy(alpha = 0.15f),
+    modifier = modifier,
+  ) {
+    Text(
+      text = text,
+      color = color,
+      fontSize = 11.sp,
+      fontWeight = FontWeight.Medium,
+      modifier = Modifier.padding(horizontal = 8.dp, vertical = 3.dp),
+    )
+  }
+}