fix: use esp_http_client for KOSync to prevent TLS OOM on ESP32-C3 (#1381)

## Summary

* **What is the goal of this PR?** Fix KOReader sync failing on HTTPS
servers due to TLS out-of-memory on ESP32-C3
* **What changes are included?**
- Replace `WiFiClientSecure`/`HTTPClient` with `esp_http_client`
(ESP-IDF native API) for all KOSync HTTP requests
- Use 2KB TLS buffers instead of the default 16KB — KOSync payloads are
tiny JSON (<1KB), so this is more than sufficient
- Use `esp_crt_bundle_attach` for proper TLS certificate verification
(replaces `setInsecure()`)
- Strip trailing slashes from server URL to prevent double-slash in API
paths (e.g. `https://server.com//users/auth`)
  - Add `lastHttpCode` static field for diagnostics
  - Add free heap logging to help debug memory issues

### Problem

The ESP32-C3 has ~46KB free heap after WiFi is initialized.
`WiFiClientSecure` allocates 16KB for TLS RX + 16KB for TLS TX = 32KB
just for the TLS buffers, leaving almost no room for the actual TLS
handshake (which needs additional dynamic allocations). This causes
KOReader sync to:

1. Fail silently with network errors on HTTPS servers (including the
default `sync.koreader.rocks`)
2. Occasionally crash with heap exhaustion

### Solution

`esp_http_client` (the ESP-IDF native HTTP client) allows configuring
`buffer_size` and `buffer_size_tx` independently. Setting both to 2KB
(total 4KB) leaves plenty of heap for the TLS handshake while still
being sufficient for KOSync's small JSON payloads.

This also fixes the `setInsecure()` anti-pattern —
`esp_crt_bundle_attach` provides proper certificate verification using
the ESP-IDF's built-in CA bundle, so credentials and reading history are
no longer sent over unverified TLS connections.

### Files changed

| File | Change |
|------|--------|
| `lib/KOReaderSync/KOReaderSyncClient.cpp` | Replace
WiFiClientSecure/HTTPClient with esp_http_client; add ResponseBuffer,
base64 encoder, createClient helper |
| `lib/KOReaderSync/KOReaderSyncClient.h` | Add `lastHttpCode` static
field |
| `lib/KOReaderSync/KOReaderCredentialStore.cpp` | Strip trailing
slashes from base URL |

## Additional Context

Tested on a CrossPoint X4 (ESP32-C3 with 4MB flash). Before this change,
KOSync auth to `sync.koreader.rocks` (HTTPS) would fail ~80% of the
time. After: works reliably.

The `base64Encode` helper is needed because `esp_http_client` doesn't
have a built-in `setAuthorization()` method like `HTTPClient` does. This
is used for the HTTP Basic Auth header required by Calibre-Web-Automated
KOSync servers.

Fixes #581

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it helps set the right
context for reviewers.

Did you use AI tools to help write this code? _**PARTIALLY**_ AI
assisted with the esp_http_client migration pattern and base64 encoder.
The root cause analysis (TLS buffer OOM) and solution design were
manual.

---------

Co-authored-by: trilwu <trilwu@users.noreply.github.com>
Co-authored-by: Justin Mitchell <justin@jmitch.com>

lib/KOReaderSync/KOReaderCredentialStore.cpp

@@ -151,16 +151,22 @@ 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) {