Explain a Backend URL/page-origin mismatch instead of a bare fetch error

A real Windows install had sync and backups fail everywhere with
"Failed to fetch" while the server itself was fully healthy - confirmed
via Invoke-WebRequest returning 200 for every endpoint the browser
couldn't reach. The page was opened via a LAN IP while Settings still
had the Backend URL set to 127.0.0.1 from an earlier session; browsers
treat those as different origins, so the server's CORS check correctly
rejected the requests, but a CORS rejection surfaces to fetch() as an
unexplained generic error with no indication of the real cause.

Settings now detects this mismatch on every render and shows the page's
actual address with a one-click button to fix it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "rc-racecore",
-  "version": "1.4.1",
+  "version": "1.4.2",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "rc-racecore",
-      "version": "1.4.1",
+      "version": "1.4.2",
       "hasInstallScript": true,
       "dependencies": {
         "better-sqlite3": "^12.11.1",