fix(node): proper CORS middleware + preflight handling

The desktop Electron renderer runs at http://127.0.0.1:5173 (dev) or
file:// (prod); the node HTTP API is at a different origin by design.
Browsers enforce CORS, and our per-handler `Access-Control-Allow-Origin: *`
header only covered the happy path — preflight OPTIONS requests, which
browsers send before any POST with a JSON body or Authorization header,
fell through to the 404 handler without CORS headers and the subsequent
real request was blocked.

Added node/cors.go — a single middleware that:
  * Sets Access-Control-Allow-Origin / -Methods / -Headers /
    -Expose-Headers / -Max-Age on every response.
  * Short-circuits OPTIONS with 204, never invoking the mux.

Wired into stats.go:ListenAndServe so the wrapping is unconditional
(the node's security model gates writes by token + Ed25519 signature,
not by origin, so wide CORS is the correct default).

Cleaned up the now-redundant per-jsonOK/jsonErr Allow-Origin setters in
api_common.go — the middleware sets a single consistent header instead
of two collisions from handlers that both write one.

Symptom before: `net::ERR_FAILED` / "CORS policy blocked" errors in
the Electron devtools console when hitting /api/* or /relay/*.
Symptom after: clean GET/POST, preflight answers in ~1ms.

node/cors.go

@@ -0,0 +1,32 @@
+package node
+
+import "net/http"
+
+// withCORS wraps any http.Handler so every response carries the CORS
+// headers browser-based clients (Electron renderer, web explorer from a
+// different origin, mobile webview) need. Also short-circuits OPTIONS
+// preflight requests with a 204 — without this, POST /api/tx with a
+// JSON body triggers a preflight that the regular handler answers as
+// 404/405 and the browser refuses the follow-up.
+//
+// The allow-list is wide on purpose. The node's security model doesn't
+// rely on same-origin — API tokens (DCHAIN_API_TOKEN + DCHAIN_API_PRIVATE)
+// and Ed25519 tx signatures are what gate writes. Cross-origin access is
+// a first-class feature here, not an attack vector.
+func withCORS(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		h := w.Header()
+		h.Set("Access-Control-Allow-Origin", "*")
+		h.Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS, HEAD, PATCH")
+		h.Set("Access-Control-Allow-Headers", "Authorization, Content-Type, X-Requested-With")
+		h.Set("Access-Control-Expose-Headers", "Content-Length, Content-Type")
+		h.Set("Access-Control-Max-Age", "86400") // cache preflight for a day
+
+		if r.Method == http.MethodOptions {
+			// Preflight. Don't hand to the mux — just answer.
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}