feat(plan): resolveCell/resolveGrid return arrays; dashboard shows up to 3

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components/PlanGrid.tsx

@@ -529,7 +529,8 @@ export default function PlanGrid({
                   </span>
                 </td>
                 {users.map((u) => {
-                  const cell = data.cells[u.id]?.[date] ?? null;
+                  const cellArr = data.cells[u.id]?.[date] ?? [];
+                  const cell = cellArr[0] ?? null;
                   const past = isPastDate(date, today);
                   // Past-day cells are read-only in the UI: an empty past
                   // cell is non-interactive (no create modal, no "+" hint),