Fix: Android compile errors in AuthScreen and MainScreen preview

- AuthScreen.kt: FocusManager is imported from androidx.compose.ui.focus,
  not androidx.compose.ui.platform (only LocalFocusManager lives there).
  The fully-qualified platform.FocusManager parameter types therefore
  failed to resolve, cascading into the clearFocus() errors.
- MainScreen.kt: MitbringslTheme gained a required sessionManager
  parameter in 729865b; the leftover template preview now passes
  SessionManager(LocalContext.current).

Verified: ./gradlew compileDebugKotlin, assembleDebug and test all green.
Note: local builds must use JDK 21 (JAVA_HOME=/usr/lib/jvm/java-21-openjdk);
the system default JDK 26 breaks AGP's JdkImageTransform (jlink) on
android-36's core-for-system-modules.jar.

android/app/src/main/java/com/example/mitbringsl/ui/auth/AuthScreen.kt

@@ -18,6 +18,7 @@ import androidx.compose.runtime.*
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Brush
+import androidx.compose.ui.focus.FocusManager
 import androidx.compose.ui.platform.LocalFocusManager
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.input.ImeAction
@@ -196,7 +197,7 @@ private fun LoggedInView(
 private fun ConnectView(
     uiState: AuthUiState,
     viewModel: AuthViewModel,
-    focusManager: androidx.compose.ui.platform.FocusManager,
+    focusManager: FocusManager,
 ) {
     Text(
         text = "Mit Server verbinden",
@@ -273,7 +274,7 @@ private fun LoginView(
     uiState: AuthUiState,
     viewModel: AuthViewModel,
     cfg: ServerAuthConfig,
-    focusManager: androidx.compose.ui.platform.FocusManager,
+    focusManager: FocusManager,
 ) {
     Text(
         text = when {

android/app/src/main/java/com/example/mitbringsl/ui/main/MainScreen.kt

@@ -4,9 +4,11 @@ import androidx.compose.foundation.layout.Column
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.navigation3.runtime.NavKey
+import com.example.mitbringsl.data.auth.SessionManager
 import com.example.mitbringsl.theme.MitbringslTheme
 
 @Composable
@@ -31,5 +33,7 @@ fun Greeting(name: String, modifier: Modifier = Modifier) {
 @Preview(showBackground = true)
 @Composable
 fun MainScreenPreview() {
-  MitbringslTheme { MainScreen(listOf("Mitbringsl App Initialized")) }
+  MitbringslTheme(sessionManager = SessionManager(LocalContext.current)) {
+    MainScreen(listOf("Mitbringsl App Initialized"))
+  }
 }