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>

lib/KOReaderSync/KOReaderDocumentId.cpp

@@ -0,0 +1,96 @@
+#include "KOReaderDocumentId.h"
+
+#include <HardwareSerial.h>
+#include <MD5Builder.h>
+#include <SDCardManager.h>
+
+namespace {
+// Extract filename from path (everything after last '/')
+std::string getFilename(const std::string& path) {
+  const size_t pos = path.rfind('/');
+  if (pos == std::string::npos) {
+    return path;
+  }
+  return path.substr(pos + 1);
+}
+}  // namespace
+
+std::string KOReaderDocumentId::calculateFromFilename(const std::string& filePath) {
+  const std::string filename = getFilename(filePath);
+  if (filename.empty()) {
+    return "";
+  }
+
+  MD5Builder md5;
+  md5.begin();
+  md5.add(filename.c_str());
+  md5.calculate();
+
+  std::string result = md5.toString().c_str();
+  Serial.printf("[%lu] [KODoc] Filename hash: %s (from '%s')\n", millis(), result.c_str(), filename.c_str());
+  return result;
+}
+
+size_t KOReaderDocumentId::getOffset(int i) {
+  // Offset = 1024 << (2*i)
+  // For i = -1: 1024 >> 2 = 256
+  // For i >= 0: 1024 << (2*i)
+  if (i < 0) {
+    return CHUNK_SIZE >> (-2 * i);
+  }
+  return CHUNK_SIZE << (2 * i);
+}
+
+std::string KOReaderDocumentId::calculate(const std::string& filePath) {
+  FsFile file;
+  if (!SdMan.openFileForRead("KODoc", filePath, file)) {
+    Serial.printf("[%lu] [KODoc] Failed to open file: %s\n", millis(), filePath.c_str());
+    return "";
+  }
+
+  const size_t fileSize = file.fileSize();
+  Serial.printf("[%lu] [KODoc] Calculating hash for file: %s (size: %zu)\n", millis(), filePath.c_str(), fileSize);
+
+  // Initialize MD5 builder
+  MD5Builder md5;
+  md5.begin();
+
+  // Buffer for reading chunks
+  uint8_t buffer[CHUNK_SIZE];
+  size_t totalBytesRead = 0;
+
+  // Read from each offset (i = -1 to 10)
+  for (int i = -1; i < OFFSET_COUNT - 1; i++) {
+    const size_t offset = getOffset(i);
+
+    // Skip if offset is beyond file size
+    if (offset >= fileSize) {
+      continue;
+    }
+
+    // Seek to offset
+    if (!file.seekSet(offset)) {
+      Serial.printf("[%lu] [KODoc] Failed to seek to offset %zu\n", millis(), offset);
+      continue;
+    }
+
+    // Read up to CHUNK_SIZE bytes
+    const size_t bytesToRead = std::min(CHUNK_SIZE, fileSize - offset);
+    const size_t bytesRead = file.read(buffer, bytesToRead);
+
+    if (bytesRead > 0) {
+      md5.add(buffer, bytesRead);
+      totalBytesRead += bytesRead;
+    }
+  }
+
+  file.close();
+
+  // Calculate final hash
+  md5.calculate();
+  std::string result = md5.toString().c_str();
+
+  Serial.printf("[%lu] [KODoc] Hash calculated: %s (from %zu bytes)\n", millis(), result.c_str(), totalBytesRead);
+
+  return result;
+}