fix: use correct signal-cli default port 8080 and robust connectivity check

- Change default noise-signal-cli-address to port 8080 (signal-cli's --http default)
- Replace getVersion (not a valid JSON-RPC method) with noisePing probe
- noise-rpc-check now returns t on any valid JSON-RPC response
- Update README to reflect correct default port

noise-rpc.el

@@ -83,10 +83,39 @@ Signals `noise-rpc-error` on JSON-RPC error or malformed response."
 
 (defun noise-rpc-check ()
   "Check connectivity to signal-cli.
-Returns the version string on success.
-Signals `noise-rpc-error` on failure."
-  (let ((result (noise-rpc-call "getVersion")))
-    (gethash "version" result)))
+Returns t if the daemon responds (even with an error indicating a valid JSON-RPC endpoint).
+Signals `noise-rpc-error` on connection failure or timeout."
+  ;; signal-cli daemon doesn't have a dedicated ping/version RPC method.
+  ;; We send a request for a purposefully invalid method. If we get back
+  ;; a valid JSON-RPC error response, the daemon is running and reachable.
+  ;; If we get a connection error, the daemon is not running.
+  (let* ((url-request-method "POST")
+         (url-request-extra-headers
+          '(("Content-Type" . "application/json")))
+         (url-request-data "{\"jsonrpc\":\"2.0\",\"method\":\"noisePing\",\"id\":0}")
+         (url (concat noise-signal-cli-address "/api/v1/rpc"))
+         (url-show-status nil)
+         (buf (condition-case nil
+                  (url-retrieve-synchronously url t)
+                (error (signal 'noise-rpc-error
+                               (list (format "Could not connect to signal-cli at %s. Is signal-cli daemon running?"
+                                             noise-signal-cli-address)))))))
+    (if (not buf)
+        (signal 'noise-rpc-error (list (format "No response from signal-cli at %s" noise-signal-cli-address)))
+      (unwind-protect
+          (with-current-buffer buf
+            (goto-char (point-min))
+            (re-search-forward "\n\n" nil t)
+            (let* ((body (buffer-substring (point) (point-max)))
+                   (parsed (condition-case nil
+                               (json-parse-string body)
+                             (error (signal 'noise-rpc-error
+                                            (list "signal-cli returned invalid JSON response"))))))
+              ;; Any valid JSON-RPC response means the daemon is alive
+              (if (or (gethash "result" parsed) (gethash "error" parsed))
+                  t
+                (signal 'noise-rpc-error (list "Invalid JSON-RPC response from signal-cli")))))
+        (kill-buffer buf)))))
 
 (provide 'noise-rpc)
 ;;; noise-rpc.el ends here