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/KOReaderCredentialStore.cpp

@@ -153,16 +153,22 @@ void KOReaderCredentialStore::setServerUrl(const std::string& url) {
 }
 
 std::string KOReaderCredentialStore::getBaseUrl() const {
+  std::string url;
   if (serverUrl.empty()) {
-    return DEFAULT_SERVER_URL;
+    url = DEFAULT_SERVER_URL;
+  } else if (serverUrl.find("://") == std::string::npos) {
+    // Normalize URL: add http:// if no protocol specified (local servers typically don't have SSL)
+    url = "http://" + serverUrl;
+  } else {
+    url = serverUrl;
   }
 
-  // Normalize URL: add http:// if no protocol specified (local servers typically don't have SSL)
-  if (serverUrl.find("://") == std::string::npos) {
-    return "http://" + serverUrl;
+  // Strip trailing slashes to avoid double-slash in API paths
+  while (!url.empty() && url.back() == '/') {
+    url.pop_back();
   }
 
-  return serverUrl;
+  return url;
 }
 
 void KOReaderCredentialStore::setMatchMethod(DocumentMatchMethod method) {