refactor(alerts/ui): fix leftover --muted refs in wizard steps

Two inline-style color refs in NotifyStep and TriggerStep were still
pointing at the undefined --muted token instead of the DS
--text-muted. Caught by the design-system-alignment verification
grep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/Alerts/RuleEditor/NotifyStep.tsx

@@ -91,7 +91,7 @@ export function NotifyStep({
           Preview rendered output
         </Button>
         {!ruleId && (
-          <p style={{ marginTop: 8, fontSize: 12, color: 'var(--muted)' }}>
+          <p style={{ marginTop: 8, fontSize: 12, color: 'var(--text-muted)' }}>
             Save the rule first to preview rendered output.
           </p>
         )}

ui/src/pages/Alerts/RuleEditor/TriggerStep.tsx

@@ -60,7 +60,7 @@ export function TriggerStep({
           Test evaluate (uses saved rule)
         </Button>
         {!ruleId && (
-          <p style={{ marginTop: 8, fontSize: 12, color: 'var(--muted)' }}>
+          <p style={{ marginTop: 8, fontSize: 12, color: 'var(--text-muted)' }}>
             Save the rule first to enable test-evaluate.
           </p>
         )}