refactor(settings): guard all Settings fields vs null; tidy import + test

Guard colorMode and glyphMode against null in the Settings compact
constructor (alongside the existing musicLevel guard), so a hand-edited
settings.json with a missing field no longer NPEs in SettingsMenu.
Remove the unused MusicLevel import from SettingsMenu. Assert that the
defaulted musicLevel survives a round-trip in SettingsStoreTest.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/src/test/java/thb/jeanluc/adventure/save/SettingsStoreTest.java

@@ -20,6 +20,7 @@ class SettingsStoreTest {
         Settings loaded = store.load();
         assertThat(loaded.colorMode()).isEqualTo(ConsoleIO.ColorMode.ON);
         assertThat(loaded.glyphMode()).isEqualTo(ConsoleIO.GlyphMode.ASCII);
+        assertThat(loaded.musicLevel()).isEqualTo(MusicLevel.MEDIUM);
     }
 
     @Test