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/KOReaderSyncClient.h

@@ -0,0 +1,59 @@
+#pragma once
+#include <string>
+
+/**
+ * Progress data from KOReader sync server.
+ */
+struct KOReaderProgress {
+  std::string document;  // Document hash
+  std::string progress;  // XPath-like progress string
+  float percentage;      // Progress percentage (0.0 to 1.0)
+  std::string device;    // Device name
+  std::string deviceId;  // Device ID
+  int64_t timestamp;     // Unix timestamp of last update
+};
+
+/**
+ * HTTP client for KOReader sync API.
+ *
+ * Base URL: https://sync.koreader.rocks:443/
+ *
+ * API Endpoints:
+ *   GET /users/auth - Authenticate (validate credentials)
+ *   GET /syncs/progress/:document - Get progress for a document
+ *   PUT /syncs/progress - Update progress for a document
+ *
+ * Authentication:
+ *   x-auth-user: username
+ *   x-auth-key: MD5 hash of password
+ */
+class KOReaderSyncClient {
+ public:
+  enum Error { OK = 0, NO_CREDENTIALS, NETWORK_ERROR, AUTH_FAILED, SERVER_ERROR, JSON_ERROR, NOT_FOUND };
+
+  /**
+   * Authenticate with the sync server (validate credentials).
+   * @return OK on success, error code on failure
+   */
+  static Error authenticate();
+
+  /**
+   * Get reading progress for a document.
+   * @param documentHash The document hash (from KOReaderDocumentId)
+   * @param outProgress Output: the progress data
+   * @return OK on success, NOT_FOUND if no progress exists, error code on failure
+   */
+  static Error getProgress(const std::string& documentHash, KOReaderProgress& outProgress);
+
+  /**
+   * Update reading progress for a document.
+   * @param progress The progress data to upload
+   * @return OK on success, error code on failure
+   */
+  static Error updateProgress(const KOReaderProgress& progress);
+
+  /**
+   * Get human-readable error message.
+   */
+  static const char* errorString(Error error);
+};