fix(desktop): Persist window size and position across restarts

The desktop window always reset to its default size after restart
because no WindowState was persisted. Add WindowStateStorage backed
by multiplatform-settings (java.util.prefs on desktop) to save and
restore width, height, and position. Use snapshotFlow inside the
Window composition to persist changes continuously, with an explicit
Preferences.flush() to ensure values reach disk before exit.

Also add 'desktop-install' Makefile target to rebuild and copy the
app to /Applications/myicek.app.

composeApp/src/desktopMain/kotlin/ch/parano/myicek/Main.kt

@@ -17,12 +17,43 @@
 
 package ch.parano.myicek
 
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.snapshotFlow
+import androidx.compose.ui.unit.dp
 import androidx.compose.ui.window.Window
+import androidx.compose.ui.window.WindowPosition
 import androidx.compose.ui.window.application
+import androidx.compose.ui.window.rememberWindowState
 import ch.parano.myicek.auth.DesktopOAuthClient
+import com.russhwolf.settings.Settings
+import java.util.prefs.Preferences
 
 fun main() = application {
-    Window(onCloseRequest = ::exitApplication, title = "MyIce") {
+    val storage = WindowStateStorage(Settings())
+    val saved = storage.load()
+    val windowState = rememberWindowState(
+        width = saved?.width?.dp ?: 1024.dp,
+        height = saved?.height?.dp ?: 768.dp,
+        position = if (saved != null) WindowPosition(saved.x.dp, saved.y.dp) else WindowPosition(0.dp, 0.dp),
+    )
+
+    Window(
+        onCloseRequest = ::exitApplication,
+        title = "MyIce",
+        state = windowState,
+    ) {
+        LaunchedEffect(Unit) {
+            snapshotFlow { windowState.size to windowState.position }
+                .collect { (size, position) ->
+                    storage.save(
+                        width = size.width.value.toInt(),
+                        height = size.height.value.toInt(),
+                        x = position.x.value.toInt(),
+                        y = position.y.value.toInt(),
+                    )
+                    Preferences.userRoot().flush()
+                }
+        }
         App(oauthClient = DesktopOAuthClient())
     }
 }