refactor: Eliminated relative path includes (#1961)

## Summary

Relative includes can hide inappropriate dependency relationships. In
this case, I found that lib/KOReaderSync/KOReaderCredentialStore.cpp was
dependent on src/JsonSettingsIO.h -- a lib -> app dependency, the
opposite direction dependencies should flow in this project.

This change replaces all relative includes with root-relative includes,
and corrects the KOReaderCredentialStore dependency by moving its JSON
settings serialization local to the KOReaderSync library.

---

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lib/KOReaderSync/KOReaderCredentialStore.cpp

@@ -6,7 +6,7 @@
 #include <ObfuscationUtils.h>
 #include <Serialization.h>
 
-#include "../../src/JsonSettingsIO.h"
+#include "KOReaderJsonIO.h"
 
 // Initialize the static instance
 KOReaderCredentialStore KOReaderCredentialStore::instance;
@@ -36,7 +36,7 @@ void legacyDeobfuscate(std::string& data) {
 
 bool KOReaderCredentialStore::saveToFile() const {
   Storage.mkdir("/.crosspoint");
-  return JsonSettingsIO::saveKOReader(*this, KOREADER_FILE_JSON);
+  return KOReaderJsonIO::save(*this, KOREADER_FILE_JSON);
 }
 
 bool KOReaderCredentialStore::loadFromFile() {
@@ -45,7 +45,7 @@ bool KOReaderCredentialStore::loadFromFile() {
     String json = Storage.readFile(KOREADER_FILE_JSON);
     if (!json.isEmpty()) {
       bool resave = false;
-      bool result = JsonSettingsIO::loadKOReader(*this, json.c_str(), &resave);
+      bool result = KOReaderJsonIO::load(*this, json.c_str(), &resave);
       if (result && resave) {
         saveToFile();
         LOG_DBG("KRS", "Resaved KOReader credentials to update format");