style(design): FINDING-002 — replace hardcoded color on triangle indicator

The billing/shipping diff triangle used inline style="color:#6b7280"
which doesn't adapt to dark mode. Now uses a CSS class with
var(--text-muted) for proper theming.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/static/css/style.css

@@ -453,6 +453,12 @@ input[type="checkbox"] {
 .fc-dark   { color: var(--text-secondary); }
 .fc-orange { color: var(--accent); }
 
+/* ── Client diff indicator (billing ≠ shipping) ──── */
+.client-diff-indicator {
+    color: var(--text-muted);
+    font-size: 0.65rem;
+}
+
 /* ── Log viewer (dark theme — keep as-is) ────────── */
 .log-viewer {
     font-family: var(--font-data);