fix(nav): placeholder de Dashboard con contenido visible

Un Surface crema vacío tras completar onboarding se confundía con una
pantalla en blanco durante pruebas en dispositivo (Huawei HBP-LX9).
Ahora muestra mascota + mensaje hasta que llegue el módulo 2.

app/app/src/main/java/mx/paputec/rachita/MainActivity.kt

@@ -6,6 +6,8 @@ import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.size
+import androidx.compose.ui.unit.dp
 import androidx.compose.material3.Surface
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
@@ -73,9 +75,25 @@ private fun RachitaNavHost() {
 
 @Composable
 private fun DashboardPlaceholder() {
-    // Intentionally empty; will be replaced when module 2 (dashboard) is wired up.
+    // Visible stand-in until module 2 lands — an all-cream screen here reads as
+    // "the app went blank" during on-device testing.
     Surface(
         modifier = Modifier.fillMaxSize(),
         color = CreamBackground,
-    ) {}
+    ) {
+        androidx.compose.foundation.layout.Column(
+            modifier = Modifier.fillMaxSize(),
+            horizontalAlignment = androidx.compose.ui.Alignment.CenterHorizontally,
+            verticalArrangement = androidx.compose.foundation.layout.Arrangement.Center,
+        ) {
+            mx.paputec.rachita.ui.onboarding.components.Mascot(
+                kind = mx.paputec.rachita.domain.model.AvatarKind.Fox,
+                modifier = Modifier.size(180.dp),
+            )
+            androidx.compose.material3.Text(
+                text = "¡Compañero elegido! El tablero de hábitos llega en el módulo 2.",
+                style = androidx.compose.material3.MaterialTheme.typography.titleLarge,
+            )
+        }
+    }
 }