Adds KOReader Sync support (#232)

## Summary

- Adds KOReader progress sync integration, allowing CrossPoint to sync
reading positions with other
KOReader-compatible devices
- Stores credentials securely with XOR obfuscation
- Uses KOReader's partial MD5 document hashing for cross-device book
matching
  - Syncs position via percentage with estimated XPath for compatibility

# Features
- Settings: KOReader Username, Password, and Authenticate options
- Sync from chapters menu: "Sync Progress" option appears when
credentials are configured
- Bidirectional sync: Can apply remote progress or upload local progress

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/main.cpp

@@ -12,6 +12,7 @@
 #include "Battery.h"
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
+#include "KOReaderCredentialStore.h"
 #include "MappedInputManager.h"
 #include "activities/boot_sleep/BootActivity.h"
 #include "activities/boot_sleep/SleepActivity.h"
@@ -289,6 +290,7 @@ void setup() {
   }
 
   SETTINGS.loadFromFile();
+  KOREADER_STORE.loadFromFile();
 
   // verify power button press duration after we've read settings.
   verifyWakeupLongPress();