fix: allow null due_date in task schema for PATCH updates

apps/web/components/tasks/task-detail-panel.tsx

@@ -82,7 +82,7 @@ export function TaskDetailPanel({
           status,
           priority,
           domain,
-          due_date: dueDate || null
+          due_date: dueDate || undefined
         })
       });
       if (!response.ok) throw new Error('Unable to save task');

packages/shared/src/schemas/task.ts

@@ -35,7 +35,7 @@ export const taskSchema = z.object({
   description: z.string().optional(),
   status: taskStatusEnum.default('todo'),
   priority: taskPriorityEnum.default('medium'),
-  due_date: z.string().datetime().optional(),
+  due_date: z.string().datetime().optional().nullable(),
   project_id: z.string().optional(),
   milestone_id: z.string().optional(),
   tags: z.array(z.string()).default([]),