Fix README and docs pages that no longer match the code

Bring the READMEs and the docs pages (en and ja) in line with the
current header:

- Fix the client sample that does not compile, the Transfer-Encoding
  statement for default headers, the verifier callback comment, the
  thread pool spawn rule, the sanitize_filename description, and the
  inverted Accept-Encoding sentence.
- Describe recent behaviour: TLS errors behind Read/Write failures,
  Host validation, Connection: close for a body without length,
  content_reader rules, refused CONNECT, WebSocketClient proxies,
  CPPHTTPLIB_INTERIM_RESPONSE_MAX_COUNT, the SSE event size limit, and
  the payload limit of the Stream API.
- Complete the API references of WebSocket, WebSocketClient and the
  stream:: helpers, and the SSE reconnect rules.
- Fix cookbook pages: keychain loading on every backend, received Close
  status, a multipart sample that trusted the client's file name, an
  echo sample that sent binary as text, SNI, pre-request handler scope,
  stop() with WebSocket handlers, and the thread pool notes.
- Restore the mutex in the LLM app's Chapter 5 code.

docs-src/pages/ja/llm-app/ch06-desktop-app.md

@@ -675,9 +675,10 @@ int main() {
 5章からの変更点をまとめると:
 
 - `#include <csignal>` → `#include <thread>`, `<cpp-embedlib-httplib.h>`, `"WebAssets.h"`, `"webview/webview.h"`
-- `signal_handler`関数を削除
+- `signal_handler`関数を削除し、`svr`を`main()`の中に移動
 - `svr.set_mount_point("/", "./public")` → `httplib::mount(svr, Web::FS)`
 - `svr.listen("127.0.0.1", 8080)` → `bind_to_any_port` + `listen_after_bind` + WebViewのイベントループ
+- macOS用に`setup_macos_edit_menu()`と`<objc/objc-runtime.h>`のインクルードを追加
 
 ハンドラのコードは1行も変わっていません。5章まで作ってきたREST API・SSEストリーミング・モデル管理がそのまま動きます。