feat(ui): add version number to start screen

app/build.gradle.kts:
- Enabled buildConfig in buildFeatures to expose VERSION_NAME

app/src/main/java/de/krisenvorrat/app/MainActivity.kt:
- Replaced plain Text greeting with centered Column layout
- Shows app title 'Krisenvorrat' (headlineLarge) and version
  'v1.0' via BuildConfig.VERSION_NAME (bodyMedium, onSurfaceVariant)

Verified: built, deployed to emulator, and confirmed via screenshot.

app/build.gradle.kts

@@ -39,6 +39,7 @@ android {
     }
     buildFeatures {
         compose = true
+        buildConfig = true
     }
 }
 

app/src/main/java/de/krisenvorrat/app/MainActivity.kt

@@ -4,11 +4,15 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+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.MaterialTheme
 import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.tooling.preview.Preview
 import dagger.hilt.android.AndroidEntryPoint
@@ -22,10 +26,23 @@ class MainActivity : ComponentActivity() {
         setContent {
             KrisenvorratTheme {
                 Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
-                    Text(
-                        text = "Krisenvorrat",
-                        modifier = Modifier.padding(innerPadding)
-                    )
+                    Column(
+                        modifier = Modifier
+                            .fillMaxSize()
+                            .padding(innerPadding),
+                        horizontalAlignment = Alignment.CenterHorizontally,
+                        verticalArrangement = Arrangement.Center
+                    ) {
+                        Text(
+                            text = "Krisenvorrat",
+                            style = MaterialTheme.typography.headlineLarge
+                        )
+                        Text(
+                            text = "v${BuildConfig.VERSION_NAME}",
+                            style = MaterialTheme.typography.bodyMedium,
+                            color = MaterialTheme.colorScheme.onSurfaceVariant
+                        )
+                    }
                 }
             }
         }