feat(settings): persist the theme through the settings binding

The palette lives in Lua, so C++ stores the name only and ui.setTheme()
writes through here before rebuilding and repainting.

native/test/fake_providers.h

@@ -26,6 +26,7 @@ struct Log : LogProvider {
 struct Settings : SettingsProvider {
   int32_t degrees = 0;
   std::string tz = "UTC0";
+  std::string themeName = "light";
   int32_t rotation() const override { return degrees; }
   Status setRotation(int32_t value) override {
     degrees = value;
@@ -36,6 +37,11 @@ struct Settings : SettingsProvider {
     tz = value;
     return Status::success();
   }
+  std::string theme() const override { return themeName; }
+  Status setTheme(const std::string& value) override {
+    themeName = value;
+    return Status::success();
+  }
 };
 
 struct Sys : SysProvider {