feat: persistent settings in LittleFS JSON

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/app_settings.h

@@ -0,0 +1,15 @@
+#pragma once
+#include <Arduino.h>
+
+struct AppSettings {
+  float altitudeM;
+  int   tzOffsetMin;
+  float lat;
+  float lon;
+};
+
+extern AppSettings settings;
+
+void settingsBegin();
+bool settingsSave();
+void settingsDefaults();