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

Keeps one writer for saved state: ui.setTheme() writes through
settings.setTheme() and then reloads the palette C cannot see.

src/host/providers_sys.cpp

@@ -42,6 +42,18 @@ Status Settings::setTimezone(const std::string& timezone) {
                          : Status::failure("cannot save settings");
 }
 
+std::string Settings::theme() const { return settings.theme.c_str(); }
+
+// Stores the name only. ui.setTheme() owns applying it, because the palette and
+// the repaint it drives exist solely in Lua.
+Status Settings::setTheme(const std::string& theme) {
+  if (theme.empty() || theme.size() > 16)
+    return Status::failure("theme must be 1 to 16 characters");
+  settings.theme = theme.c_str();
+  return settings.save() ? Status::success()
+                         : Status::failure("cannot save settings");
+}
+
 int32_t Sys::millis() const { return static_cast<int32_t>(::millis()); }
 
 esp32lua::MemoryInfo Sys::memory() const {