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/commonTest/kotlin/ch/parano/myicek/WindowStateStorageTest.kt

@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// MyIce Kotlin Multiplatform — schedule/convocation viewer
+// Copyright (C) 2026  parano.ch
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package ch.parano.myicek
+
+import com.russhwolf.settings.MapSettings
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+class WindowStateStorageTest {
+
+    @Test
+    fun saveAndLoadWindowState() {
+        val settings = MapSettings()
+        val storage = WindowStateStorage(settings)
+
+        storage.save(width = 1200, height = 800, x = 100, y = 50)
+
+        val state = storage.load()
+        assertEquals(1200, state?.width)
+        assertEquals(800, state?.height)
+        assertEquals(100, state?.x)
+        assertEquals(50, state?.y)
+    }
+
+    @Test
+    fun loadReturnsNullWhenNothingSaved() {
+        val settings = MapSettings()
+        val storage = WindowStateStorage(settings)
+
+        assertNull(storage.load())
+    }
+
+    @Test
+    fun saveOverwritesPreviousState() {
+        val settings = MapSettings()
+        val storage = WindowStateStorage(settings)
+
+        storage.save(width = 1200, height = 800, x = 100, y = 50)
+        storage.save(width = 1024, height = 768, x = 200, y = 75)
+
+        val state = storage.load()
+        assertEquals(1024, state?.width)
+        assertEquals(768, state?.height)
+        assertEquals(200, state?.x)
+        assertEquals(75, state?.y)
+    }
+}