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.

---

### 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? _**PARTIALLY**_

lib/KOReaderSync/KOReaderCredentialStore.h

@@ -8,12 +8,6 @@ enum class DocumentMatchMethod : uint8_t {
   BINARY = 1,    // Match by partial MD5 of file content (more accurate, but files must be identical)
 };
 
-class KOReaderCredentialStore;
-namespace JsonSettingsIO {
-bool saveKOReader(const KOReaderCredentialStore& store, const char* path);
-bool loadKOReader(KOReaderCredentialStore& store, const char* json, bool* needsResave);
-}  // namespace JsonSettingsIO
-
 /**
  * Singleton class for storing KOReader sync credentials on the SD card.
  * Passwords are XOR-obfuscated with the device's unique hardware MAC address
@@ -33,9 +27,6 @@ class KOReaderCredentialStore {
 
   bool loadFromBinaryFile();
 
-  friend bool JsonSettingsIO::saveKOReader(const KOReaderCredentialStore&, const char*);
-  friend bool JsonSettingsIO::loadKOReader(KOReaderCredentialStore&, const char*, bool*);
-
  public:
   // Delete copy constructor and assignment
   KOReaderCredentialStore(const KOReaderCredentialStore&) = delete;