fix: render basis with explicit sign and absolute delta

Format basis row as +/-$abs(delta) to match the spec and avoid
awkward %+s formatting behavior.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/services/tele/view/price.go

@@ -2,6 +2,7 @@ package view
 
 import (
 	"fmt"
+	"math"
 	"strings"
 	"time"
 
@@ -102,7 +103,11 @@ func RenderRichTokenMessage(in RichTokenMessageInput) string {
 	if in.HasSpot && in.HasFuture && in.SpotPrice > 0 {
 		delta := in.FuturePrice - in.SpotPrice
 		deltaPct := delta / in.SpotPrice * 100
-		rows = append(rows, fmt.Sprintf("🧭 Basis: $%+s (%+.2f%%)", RenderPrice(delta), deltaPct))
+		sign := "+"
+		if delta < 0 {
+			sign = "-"
+		}
+		rows = append(rows, fmt.Sprintf("🧭 Basis: %s$%s (%+.2f%%)", sign, RenderPrice(math.Abs(delta)), deltaPct))
 	}
 
 	if in.HasFuture {