Fix wallet connect signature mismatch on mobile and desktop.

Align backend EIP-191 auth message with the DBIS Explorer text the frontend and legacy SPA already sign, instead of the stale SolaceScan string.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/services/api/access.ts

@@ -166,6 +166,7 @@ function setStoredWalletSession(session: WalletAccessSession | null) {
   window.dispatchEvent(new Event(ACCESS_SESSION_EVENT))
 }
 
+// Keep in sync with walletAuthSignMessage() in backend/auth/wallet_auth.go.
 function buildWalletMessage(nonce: string) {
   return `Sign this message to authenticate with DBIS Explorer.\n\nNonce: ${nonce}`
 }