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/activities/settings/KOReaderAuthActivity.h

@@ -0,0 +1,44 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+
+#include "activities/ActivityWithSubactivity.h"
+
+/**
+ * Activity for testing KOReader credentials.
+ * Connects to WiFi and authenticates with the KOReader sync server.
+ */
+class KOReaderAuthActivity final : public ActivityWithSubactivity {
+ public:
+  explicit KOReaderAuthActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                const std::function<void()>& onComplete)
+      : ActivityWithSubactivity("KOReaderAuth", renderer, mappedInput), onComplete(onComplete) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  bool preventAutoSleep() override { return state == CONNECTING || state == AUTHENTICATING; }
+
+ private:
+  enum State { WIFI_SELECTION, CONNECTING, AUTHENTICATING, SUCCESS, FAILED };
+
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  bool updateRequired = false;
+
+  State state = WIFI_SELECTION;
+  std::string statusMessage;
+  std::string errorMessage;
+
+  const std::function<void()> onComplete;
+
+  void onWifiSelectionComplete(bool success);
+  void performAuthentication();
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render();
+};