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/KOReaderCredentialStore.h

@@ -0,0 +1,69 @@
+#pragma once
+#include <cstdint>
+#include <string>
+
+// Document matching method for KOReader sync
+enum class DocumentMatchMethod : uint8_t {
+  FILENAME = 0,  // Match by filename (simpler, works across different file sources)
+  BINARY = 1,    // Match by partial MD5 of file content (more accurate, but files must be identical)
+};
+
+/**
+ * Singleton class for storing KOReader sync credentials on the SD card.
+ * Credentials are stored in /sd/.crosspoint/koreader.bin with basic
+ * XOR obfuscation to prevent casual reading (not cryptographically secure).
+ */
+class KOReaderCredentialStore {
+ private:
+  static KOReaderCredentialStore instance;
+  std::string username;
+  std::string password;
+  std::string serverUrl;                                            // Custom sync server URL (empty = default)
+  DocumentMatchMethod matchMethod = DocumentMatchMethod::FILENAME;  // Default to filename for compatibility
+
+  // Private constructor for singleton
+  KOReaderCredentialStore() = default;
+
+  // XOR obfuscation (symmetric - same for encode/decode)
+  void obfuscate(std::string& data) const;
+
+ public:
+  // Delete copy constructor and assignment
+  KOReaderCredentialStore(const KOReaderCredentialStore&) = delete;
+  KOReaderCredentialStore& operator=(const KOReaderCredentialStore&) = delete;
+
+  // Get singleton instance
+  static KOReaderCredentialStore& getInstance() { return instance; }
+
+  // Save/load from SD card
+  bool saveToFile() const;
+  bool loadFromFile();
+
+  // Credential management
+  void setCredentials(const std::string& user, const std::string& pass);
+  const std::string& getUsername() const { return username; }
+  const std::string& getPassword() const { return password; }
+
+  // Get MD5 hash of password for API authentication
+  std::string getMd5Password() const;
+
+  // Check if credentials are set
+  bool hasCredentials() const;
+
+  // Clear credentials
+  void clearCredentials();
+
+  // Server URL management
+  void setServerUrl(const std::string& url);
+  const std::string& getServerUrl() const { return serverUrl; }
+
+  // Get base URL for API calls (with http:// normalization if no protocol, falls back to default)
+  std::string getBaseUrl() const;
+
+  // Document matching method
+  void setMatchMethod(DocumentMatchMethod method);
+  DocumentMatchMethod getMatchMethod() const { return matchMethod; }
+};
+
+// Helper macro to access credential store
+#define KOREADER_STORE KOReaderCredentialStore::getInstance()