fix: Auto calculate the settings size on serialization (#832)

## Summary

* The constant SETTINGS_CONST was hardcoded and needed to be updated
whenever an additional setting was added
* This is no longer necessary as the settings size will be determined
automatically on settings persistence

## Additional Context

* New settings need to be added (as previously) in saveToFile - that's
it

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? YES

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

src/CrossPointSettings.cpp

@@ -5,6 +5,7 @@
 #include <Serialization.h>
 
 #include <cstring>
+#include <string>
 
 #include "fontIds.h"
 
@@ -21,8 +22,7 @@ void readAndValidate(FsFile& file, uint8_t& member, const uint8_t maxValue) {
 
 namespace {
 constexpr uint8_t SETTINGS_FILE_VERSION = 1;
-// Increment this when adding new persisted settings fields
-constexpr uint8_t SETTINGS_COUNT = 30;
+// SETTINGS_COUNT is now calculated automatically in saveToFile
 constexpr char SETTINGS_FILE[] = "/.crosspoint/settings.bin";
 
 // Validate front button mapping to ensure each hardware button is unique.
@@ -77,6 +77,68 @@ void applyLegacyFrontButtonLayout(CrossPointSettings& settings) {
 }
 }  // namespace
 
+class SettingsWriter {
+ public:
+  bool is_counting = false;
+  uint8_t item_count = 0;
+  template <typename T>
+
+  void writeItem(FsFile& file, const T& value) {
+    if (is_counting) {
+      item_count++;
+    } else {
+      serialization::writePod(file, value);
+    }
+  }
+
+  void writeItemString(FsFile& file, const char* value) {
+    if (is_counting) {
+      item_count++;
+    } else {
+      serialization::writeString(file, std::string(value));
+    }
+  }
+};
+
+uint8_t CrossPointSettings::writeSettings(FsFile& file, bool count_only) const {
+  SettingsWriter writer;
+  writer.is_counting = count_only;
+
+  writer.writeItem(file, sleepScreen);
+  writer.writeItem(file, extraParagraphSpacing);
+  writer.writeItem(file, shortPwrBtn);
+  writer.writeItem(file, statusBar);
+  writer.writeItem(file, orientation);
+  writer.writeItem(file, frontButtonLayout);  // legacy
+  writer.writeItem(file, sideButtonLayout);
+  writer.writeItem(file, fontFamily);
+  writer.writeItem(file, fontSize);
+  writer.writeItem(file, lineSpacing);
+  writer.writeItem(file, paragraphAlignment);
+  writer.writeItem(file, sleepTimeout);
+  writer.writeItem(file, refreshFrequency);
+  writer.writeItem(file, screenMargin);
+  writer.writeItem(file, sleepScreenCoverMode);
+  writer.writeItemString(file, opdsServerUrl);
+  writer.writeItem(file, textAntiAliasing);
+  writer.writeItem(file, hideBatteryPercentage);
+  writer.writeItem(file, longPressChapterSkip);
+  writer.writeItem(file, hyphenationEnabled);
+  writer.writeItemString(file, opdsUsername);
+  writer.writeItemString(file, opdsPassword);
+  writer.writeItem(file, sleepScreenCoverFilter);
+  writer.writeItem(file, uiTheme);
+  writer.writeItem(file, frontButtonBack);
+  writer.writeItem(file, frontButtonConfirm);
+  writer.writeItem(file, frontButtonLeft);
+  writer.writeItem(file, frontButtonRight);
+  writer.writeItem(file, fadingFix);
+  writer.writeItem(file, embeddedStyle);
+  // New fields need to be added at end for backward compatibility
+
+  return writer.item_count;
+}
+
 bool CrossPointSettings::saveToFile() const {
   // Make sure the directory exists
   Storage.mkdir("/.crosspoint");
@@ -86,39 +148,15 @@ bool CrossPointSettings::saveToFile() const {
     return false;
   }
 
+  // First pass: count the items
+  uint8_t item_count = writeSettings(outputFile, true);  // This will just count, not write
+
+  // Write header
   serialization::writePod(outputFile, SETTINGS_FILE_VERSION);
-  serialization::writePod(outputFile, SETTINGS_COUNT);
-  serialization::writePod(outputFile, sleepScreen);
-  serialization::writePod(outputFile, extraParagraphSpacing);
-  serialization::writePod(outputFile, shortPwrBtn);
-  serialization::writePod(outputFile, statusBar);
-  serialization::writePod(outputFile, orientation);
-  serialization::writePod(outputFile, frontButtonLayout);  // legacy
-  serialization::writePod(outputFile, sideButtonLayout);
-  serialization::writePod(outputFile, fontFamily);
-  serialization::writePod(outputFile, fontSize);
-  serialization::writePod(outputFile, lineSpacing);
-  serialization::writePod(outputFile, paragraphAlignment);
-  serialization::writePod(outputFile, sleepTimeout);
-  serialization::writePod(outputFile, refreshFrequency);
-  serialization::writePod(outputFile, screenMargin);
-  serialization::writePod(outputFile, sleepScreenCoverMode);
-  serialization::writeString(outputFile, std::string(opdsServerUrl));
-  serialization::writePod(outputFile, textAntiAliasing);
-  serialization::writePod(outputFile, hideBatteryPercentage);
-  serialization::writePod(outputFile, longPressChapterSkip);
-  serialization::writePod(outputFile, hyphenationEnabled);
-  serialization::writeString(outputFile, std::string(opdsUsername));
-  serialization::writeString(outputFile, std::string(opdsPassword));
-  serialization::writePod(outputFile, sleepScreenCoverFilter);
-  serialization::writePod(outputFile, uiTheme);
-  serialization::writePod(outputFile, frontButtonBack);
-  serialization::writePod(outputFile, frontButtonConfirm);
-  serialization::writePod(outputFile, frontButtonLeft);
-  serialization::writePod(outputFile, frontButtonRight);
-  serialization::writePod(outputFile, fadingFix);
-  serialization::writePod(outputFile, embeddedStyle);
-  // New fields added at end for backward compatibility
+  serialization::writePod(outputFile, static_cast<uint8_t>(item_count));
+  // Second pass: actually write the settings
+  writeSettings(outputFile);  // This will write the actual data
+
   outputFile.close();
 
   LOG_DBG("CPS", "Settings saved to file");