feat(tasks): add configurable process variables display on task cards

[javim91]

• Add Variables section to FilterModal: define which process variables appear on each task card, with showUndefinedVariable toggle and persistence to filter properties.
• Enrich tasks with variables via single batch /variable-instance call on task list fetch.
• Display variables in TasksNavBar per task: collapse/expand with semi-transparent gradient overlay, date formatting
  using user preferences, object type shows simple class name, full label/value tooltip on hover.
• Add FilterVariable model and fix FilterProperties silently dropping variables field.