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>

backend/auth/wallet_auth.go

@@ -110,6 +110,12 @@ type WalletAuthResponse struct {
 	InstitutionName   string             `json:"institution_name,omitempty"`
 }
 
+// walletAuthSignMessage returns the EIP-191 plaintext users sign during wallet login.
+// Must stay in sync with frontend buildWalletMessage() in access.ts and explorer-spa.js.
+func walletAuthSignMessage(nonce string) string {
+	return fmt.Sprintf("Sign this message to authenticate with DBIS Explorer.\n\nNonce: %s", nonce)
+}
+
 // GenerateNonce generates a random nonce for wallet authentication
 func (w *WalletAuth) GenerateNonce(ctx context.Context, address string) (*NonceResponse, error) {
 	// Validate address format
@@ -184,7 +190,7 @@ func (w *WalletAuth) AuthenticateWallet(ctx context.Context, req *WalletAuthRequ
 	}
 
 	// Verify signature
-	message := fmt.Sprintf("Sign this message to authenticate with SolaceScan.\n\nNonce: %s", req.Nonce)
+	message := walletAuthSignMessage(req.Nonce)
 	messageHash := accounts.TextHash([]byte(message))
 
 	sigBytes, err := decodeWalletSignature(req.Signature)