fix: use esp_http_client for KOSync to prevent TLS OOM on ESP32-C3

The Arduino WiFiClientSecure allocates 16KB TLS buffers by default,
which exhausts the ESP32-C3's limited heap (~46KB free after WiFi)
during the TLS handshake. This causes KOReader sync to fail silently
or crash on HTTPS servers (including the default sync.koreader.rocks).

Replace WiFiClientSecure/HTTPClient with esp_http_client (ESP-IDF),
which supports configurable buffer sizes. Use 2KB TLS buffers — more
than sufficient for KOSync's tiny JSON payloads (<1KB).

Also:
- Use esp_crt_bundle for proper TLS certificate verification instead
  of setInsecure()
- Strip trailing slashes from server URL to prevent double-slash in
  API paths
- Add lastHttpCode for diagnostics
- Add heap logging to help debug memory issues

Fixes #581

(cherry picked from commit 835abc19ff14fcbea8571ccb0dfd8d21e882b84c)

lib/KOReaderSync/KOReaderSyncClient.h

@@ -74,4 +74,7 @@ class KOReaderSyncClient {
    * Get human-readable error message.
    */
   static const char* errorString(Error error);
+
+  /** HTTP status code from the last request (for diagnostics). */
+  static int lastHttpCode;
 };