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/handlers/api.go

@@ -145,35 +145,9 @@ func (hdlr *Handlers) handleCreateSession(
 	writer http.ResponseWriter,
 	request *http.Request,
 ) {
-	// Validate hashcash proof-of-work if configured.
-	if hdlr.params.Config.HashcashBits > 0 {
-		stamp := request.Header.Get("X-Hashcash")
-		if stamp == "" {
-			hdlr.respondError(
-				writer, request,
-				"hashcash proof-of-work required",
-				http.StatusPaymentRequired,
-			)
-
-			return
-		}
-
-		err := hdlr.hashcashVal.Validate(
-			stamp, hdlr.params.Config.HashcashBits,
-		)
-		if err != nil {
-			hdlr.respondError(
-				writer, request,
-				"invalid hashcash stamp: "+err.Error(),
-				http.StatusPaymentRequired,
-			)
-
-			return
-		}
-	}
-
 	type createRequest struct {
-		Nick string `json:"nick"`
+		Nick     string `json:"nick"`
+		Hashcash string `json:"hashcash,omitempty"`
 	}
 
 	var payload createRequest
@@ -189,6 +163,32 @@ func (hdlr *Handlers) handleCreateSession(
 		return
 	}
 
+	// Validate hashcash proof-of-work if configured.
+	if hdlr.params.Config.HashcashBits > 0 {
+		if payload.Hashcash == "" {
+			hdlr.respondError(
+				writer, request,
+				"hashcash proof-of-work required",
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+
+		err = hdlr.hashcashVal.Validate(
+			payload.Hashcash, hdlr.params.Config.HashcashBits,
+		)
+		if err != nil {
+			hdlr.respondError(
+				writer, request,
+				"invalid hashcash stamp: "+err.Error(),
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+	}
+
 	payload.Nick = strings.TrimSpace(payload.Nick)
 
 	if !validNickRe.MatchString(payload.Nick) {