Upgrade to new Window API; set minimum window size

src/main/kotlin/org/briarproject/briar/desktop/BriarService.kt

 package org.briarproject.briar.desktop
 
-import androidx.compose.desktop.Window
 import androidx.compose.foundation.background
 import androidx.compose.material.MaterialTheme
 import androidx.compose.runtime.Composable
@@ -11,6 +10,7 @@ import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.window.Window
 import org.briarproject.bramble.api.account.AccountManager
 import org.briarproject.bramble.api.contact.Contact
 import org.briarproject.bramble.api.contact.ContactManager
@@ -24,9 +24,11 @@ import org.briarproject.briar.desktop.dialogs.Registration
 import org.briarproject.briar.desktop.paul.theme.DarkColorPallet
 import org.briarproject.briar.desktop.paul.theme.briarBlack
 import org.briarproject.briar.desktop.paul.views.BriarUIStateManager
+import java.awt.Dimension
 import javax.annotation.concurrent.Immutable
 import javax.inject.Inject
 import javax.inject.Singleton
+import kotlin.system.exitProcess
 
 enum class Screen {
     REGISTRATION,
@@ -81,7 +83,11 @@ constructor(
                 }
             )
         }
-        Window(title = title) {
+        Window(
+            title = title,
+            onCloseRequest = { exitProcess(0) },
+        ) {
+            window.minimumSize = Dimension(800, 600)
             MaterialTheme(colors = DarkColorPallet) {
                 when (screenState) {
                     Screen.REGISTRATION ->