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_test.go

@@ -118,6 +118,20 @@ func TestRenderRichTokenMessage_SpotAndFuture(t *testing.T) {
 		MarginAPRPercent: 5.11,
 	})
 
-	assertContainsAll(t, msg, "🪙 SOL", "💵 Spot:", "📈 Future:", "🧭 Basis:", "💸 Funding:", "🏦 Margin:")
+	assertContainsAll(t, msg, "🪙 SOL", "💵 Spot:", "📈 Future:", "🧭 Basis: +$0.6000", "💸 Funding:", "🏦 Margin:")
 	assertContainsNone(t, msg, "🅰️ Alpha:", "📊 Alpha 24h:")
 }
+
+func TestRenderRichTokenMessage_SpotAndFutureNegativeBasis(t *testing.T) {
+	msg := RenderRichTokenMessage(RichTokenMessageInput{
+		Token:         "ADA",
+		HasSpot:       true,
+		SpotPrice:     1.2,
+		HasFuture:     true,
+		FuturePrice:   1.1,
+		FundingRate:   0.000011,
+		FundingTimeMs: 1740000000000,
+	})
+
+	assertContainsAll(t, msg, "🧭 Basis: -$0.10000 (-8.33%)")
+}