fix(kanban): infinite ref-merge loop + drag lag

Two issues:

1. "Maximum update depth exceeded" inside Mantine's useMergedRef during
   drag. Root cause: the `data` object passed to dnd-kit's `useSortable`
   in KanbanCard and KanbanColumn was re-created on every render. Mantine
   Paper composes its internal ref with the consumer ref via
   useMergedRef, which uses a useCallback whose deps array contains the
   refs themselves. Whenever the underlying setNodeRef from useSortable
   became unstable (because dnd-kit's internal state churned on data
   identity change), the merged ref was reassigned each commit -> setState
   inside the ref callback -> next render -> new data identity -> loop.
   Wrap the sortable data in useMemo keyed on its real inputs.

2. Drag feels laggy. Each card listens to a 1-second `now` clock that
   re-renders the entire board to refresh the "time in column" label.
   Pause that interval while a drag is active so dnd-kit's per-pixel
   reconciliation does not also re-mount/re-layout every card every
   second. Tick resumes the moment the drag ends.

Also move the Select `data` array for the column max-time popover from
an inline expression to a module-level constant; same array identity
across all column instances and renders -> Mantine Combobox stops
re-running its diffing effect for free.

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

frontend/src/App.tsx

@@ -300,10 +300,14 @@ export function App() {
     reloadRequesters();
   }, [reloadTags, reloadRequesters]);
 
+  // Tick de reloj para "tiempo en columna" en cards. Pausamos durante drag
+  // porque dispara re-render de TODAS las cards cada segundo y el drag de
+  // dnd-kit sufre tirones serios con muchos elementos.
   useEffect(() => {
+    if (activeCard || activeColumnId) return;
     const t = setInterval(() => setNow(Date.now()), 1000);
     return () => clearInterval(t);
-  }, []);
+  }, [activeCard, activeColumnId]);
 
   useEffect(() => {
     const t = setInterval(() => {