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/components/KanbanCard.tsx

@@ -32,7 +32,7 @@ import {
   IconUserCircle,
 } from "@tabler/icons-react";
 import { DatePickerInput } from "@mantine/dates";
-import { memo, useCallback, useEffect, useRef, useState } from "react";
+import { memo, useCallback, useEffect, useMemo, useRef, useState } from "react";
 import type { Card, CardColor, User } from "../types";
 import { colorBg, colorBorder, tagColor } from "./colors";
 import { ColorPickerGrid } from "./ColorPickerGrid";
@@ -101,9 +101,17 @@ function KanbanCardImpl({
   const cardElRef = useRef<HTMLElement | null>(null);
   const draggingStickerRef = useRef<number | null>(null);
   const stickerMode = !!activeSticker;
+  // Memo: useSortable es sensible a la identidad del objeto `data`. Si lo
+  // re-creamos cada render, el setNodeRef interno se vuelve inestable y
+  // dispara loops por useMergedRef de Mantine (Paper). Issue: maximum
+  // update depth visto durante drag.
+  const sortableData = useMemo(
+    () => ({ type: "card" as const, columnId: card.column_id, locked: card.locked }),
+    [card.column_id, card.locked]
+  );
   const { attributes, listeners, setNodeRef, transform, transition, isDragging } = useSortable({
     id: card.id,
-    data: { type: "card", columnId: card.column_id, locked: card.locked },
+    data: sortableData,
     disabled: stickerMode,
   });