client/web: add csrf protection to web client api

Adds csrf protection and hooks up an initial POST request from
the React web client.

Updates tailscale/corp#13775

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/src/api.ts

@@ -0,0 +1,32 @@
+let csrfToken: string
+
+// apiFetch wraps the standard JS fetch function
+// with csrf header management.
+export function apiFetch(
+  input: RequestInfo | URL,
+  init?: RequestInit | undefined
+): Promise<Response> {
+  return fetch(input, {
+    ...init,
+    headers: withCsrfToken(init?.headers),
+  }).then((r) => {
+    updateCsrfToken(r)
+    if (!r.ok) {
+      return r.text().then((err) => {
+        throw new Error(err)
+      })
+    }
+    return r
+  })
+}
+
+function withCsrfToken(h?: HeadersInit): HeadersInit {
+  return { ...h, "X-CSRF-Token": csrfToken }
+}
+
+function updateCsrfToken(r: Response) {
+  const tok = r.headers.get("X-CSRF-Token")
+  if (tok) {
+    csrfToken = tok
+  }
+}