Add Cookbook C01-C19 (draft)

docs-src/pages/en/cookbook/c11-progress-callback.md

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+---
+title: "C11. Use the Progress Callback"
+order: 11
+status: "draft"
+---
+
+To display download or upload progress, pass a `DownloadProgress` or `UploadProgress` callback. Both take two arguments: `(current, total)`.
+
+## Download progress
+
+```cpp
+httplib::Client cli("http://localhost:8080");
+
+auto res = cli.Get("/large-file",
+  [](size_t current, size_t total) {
+    auto percent = (total > 0) ? (current * 100 / total) : 0;
+    std::cout << "\rDownloading: " << percent << "% ("
+              << current << "/" << total << ")" << std::flush;
+    return true; // return false to abort
+  });
+std::cout << std::endl;
+```
+
+The callback fires each time data arrives. `total` comes from the Content-Length header — if the server doesn't send one, it may be `0`. In that case, you can't compute a percentage, so just display bytes received.
+
+## Upload progress
+
+Uploads work the same way. Pass an `UploadProgress` as the last argument to `Post()` or `Put()`.
+
+```cpp
+httplib::Client cli("http://localhost:8080");
+
+std::string body = load_large_body();
+
+auto res = cli.Post("/upload", body, "application/octet-stream",
+  [](size_t current, size_t total) {
+    auto percent = current * 100 / total;
+    std::cout << "\rUploading: " << percent << "%" << std::flush;
+    return true;
+  });
+std::cout << std::endl;
+```
+
+## Cancel mid-transfer
+
+Return `false` from the callback to abort the transfer. This is how you wire up a "Cancel" button in a UI — flip a flag, and the next progress tick stops the transfer.
+
+```cpp
+std::atomic<bool> cancelled{false};
+
+auto res = cli.Get("/large-file",
+  [&](size_t current, size_t total) {
+    return !cancelled.load();
+  });
+```
+
+> **Note:** `ContentReceiver` and the progress callback can be used together. When you want to stream to a file and show progress at the same time, pass both.
+
+> For a concrete example of saving to a file, see C01. Get the Response Body / Save to a File.