feat(card): show scheduled date range next to dated subtasks

When a subtask has start_date or target_date set, the card
detail's checklist row now shows a small '5 Mar → 19 Mar' label
next to the title. Mirrors the Gantt's view of the same data
without duplicating the chart on the card page.

Build clean.

src/components/card/subtask-list.tsx

@@ -99,6 +99,14 @@ export function SubtaskList({
             >
               {subtask.title}
             </span>
+            {(subtask.start_date || subtask.target_date) && (
+              <span
+                className="shrink-0 text-xs tabular-nums text-text-muted"
+                aria-label={`Scheduled ${subtask.start_date || "?"} to ${subtask.target_date || "?"}`}
+              >
+                {formatRange(subtask.start_date, subtask.target_date)}
+              </span>
+            )}
             {editable && (
               <button
                 type="button"
@@ -136,4 +144,32 @@ export function SubtaskList({
       )}
     </section>
   );
+}
+
+const MONTHS = [
+  "Jan",
+  "Feb",
+  "Mar",
+  "Apr",
+  "May",
+  "Jun",
+  "Jul",
+  "Aug",
+  "Sep",
+  "Oct",
+  "Nov",
+  "Dec",
+];
+
+function short(iso: string): string {
+  if (!iso) return "?";
+  const month = Number(iso.slice(5, 7));
+  if (!Number.isFinite(month) || month < 1 || month > 12) return iso;
+  return `${Number(iso.slice(8, 10))} ${MONTHS[month - 1]}`;
+}
+
+function formatRange(start: string, end: string): string {
+  if (start && end) return `${short(start)} → ${short(end)}`;
+  if (end) return `by ${short(end)}`;
+  return `from ${short(start)}`;
 }