refactor: move hashcash stamp from X-Hashcash header to JSON request body

Move the hashcash proof-of-work stamp from the X-Hashcash HTTP header
into the JSON request body as a 'hashcash' field on POST /api/v1/session.

Updated server handler, CLI client, SPA client, and documentation.

internal/cli/api/client.go

@@ -52,7 +52,7 @@ func (client *Client) CreateSession(
 	// Fetch server info to check for hashcash requirement.
 	info, err := client.GetServerInfo()
 
-	var headers map[string]string
+	var hashcashStamp string
 
 	if err == nil && info.HashcashBits > 0 {
 		resource := info.Name
@@ -60,17 +60,13 @@ func (client *Client) CreateSession(
 			resource = "neoirc"
 		}
 
-		stamp := MintHashcash(info.HashcashBits, resource)
-		headers = map[string]string{
-			"X-Hashcash": stamp,
-		}
+		hashcashStamp = MintHashcash(info.HashcashBits, resource)
 	}
 
-	data, err := client.doWithHeaders(
+	data, err := client.do(
 		http.MethodPost,
 		"/api/v1/session",
-		&SessionRequest{Nick: nick},
-		headers,
+		&SessionRequest{Nick: nick, Hashcash: hashcashStamp},
 	)
 	if err != nil {
 		return nil, err