Add Cookbook C01-C19 (draft)

docs-src/pages/en/cookbook/c03-default-headers.md

@@ -0,0 +1,55 @@
+---
+title: "C03. Set Default Headers"
+order: 3
+status: "draft"
+---
+
+When you want the same headers on every request, use `set_default_headers()`. Once set, they're attached automatically to every request sent from that client.
+
+## Basic usage
+
+```cpp
+httplib::Client cli("https://api.example.com");
+
+cli.set_default_headers({
+  {"Accept", "application/json"},
+  {"User-Agent", "my-app/1.0"},
+});
+
+auto res = cli.Get("/users");
+```
+
+Register the headers you need on every API call — like `Accept` or `User-Agent` — in one place. No need to repeat them on each request.
+
+## Send a Bearer token on every request
+
+```cpp
+httplib::Client cli("https://api.example.com");
+
+cli.set_default_headers({
+  {"Authorization", "Bearer " + token},
+  {"Accept", "application/json"},
+});
+
+auto res1 = cli.Get("/me");
+auto res2 = cli.Get("/projects");
+```
+
+Set the auth token once, and every subsequent request carries it. Handy when you're writing an API client that hits multiple endpoints.
+
+> **Note:** `set_default_headers()` **replaces** the existing default headers. Even if you only want to add one, pass the full set again.
+
+## Combine with per-request headers
+
+You can still pass extra headers on individual requests, even with defaults set.
+
+```cpp
+httplib::Headers headers = {
+  {"X-Request-ID", "abc-123"},
+};
+auto res = cli.Get("/users", headers);
+```
+
+Per-request headers are **added** on top of the defaults. Both are sent to the server.
+
+> For details on Bearer token auth, see C06. Call an API with a Bearer Token.