Introduce StateManager class for persisting state to EEPROM.

examples/Gravity/app_state.h

@@ -0,0 +1,18 @@
+#ifndef APP_STATE_H
+#define APP_STATE_H
+
+#include <gravity.h>
+#include "channel.h"
+
+// Global state for settings and app behavior.
+struct AppState {
+    int tempo = 120;
+    bool refresh_screen = true;
+    bool editing_param = false;
+    int selected_param = 0;
+    byte selected_channel = 0;  // 0=tempo, 1-6=output channel
+    Source selected_source = SOURCE_INTERNAL;
+    Channel channel[OUTPUT_COUNT];
+};
+
+#endif // APP_STATE_H