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/en/llm-app/ch06-desktop-app.md

@@ -675,9 +675,10 @@ int main() {
 To summarize the changes from Chapter 5:
 
 - `#include <csignal>` replaced with `#include <thread>`, `<cpp-embedlib-httplib.h>`, `"WebAssets.h"`, `"webview/webview.h"`
-- Removed the `signal_handler` function
+- Removed the `signal_handler` function and moved `svr` into `main()`
 - `svr.set_mount_point("/", "./public")` replaced with `httplib::mount(svr, Web::FS)`
 - `svr.listen("127.0.0.1", 8080)` replaced with `bind_to_any_port` + `listen_after_bind` + WebView event loop
+- Added `setup_macos_edit_menu()` and its `<objc/objc-runtime.h>` include (macOS only)
 
 Not a single line of handler code has changed. The REST API, SSE streaming, and model management built through Chapter 5 all work as-is.