fix(shared): align todo contracts with the wire format the client sends

The optional metadata fields (category, due_date, description,
assigned_user_id — plus priority on update) now accept explicit null: the
client clears fields by sending null, which the schemas rejected. checked
accepts the legacy 0/1 numeric form alongside boolean (the integration net
pins it). Drops the client-side casts that papered over the too-narrow
types.

client/src/components/Todo/TodoListPanel.tsx

@@ -350,7 +350,7 @@ function DetailPane({ item, tripId, categories, members, onClose }: {
         name: name.trim(), description: desc || null,
         due_date: dueDate || null, category: category || null,
         assigned_user_id: assignedUserId, priority,
-      } as any)
+      })
     } catch (err: unknown) { toast.error(err instanceof Error ? err.message : t('common.error')) }
     setSaving(false)
   }
@@ -566,7 +566,7 @@ function NewTaskPane({ tripId, categories, members, defaultCategory, onCreated,
         name: name.trim(), description: desc || null, priority,
         due_date: dueDate || null, category: trimmedCategory || null,
         assigned_user_id: assignedUserId,
-      } as any)
+      })
       if (item?.id) onCreated(item.id)
     } catch (err: unknown) { toast.error(err instanceof Error ? err.message : t('common.error')) }
     setSaving(false)

client/src/mobile/screens/trip/tabs/MTaskSheet.tsx

@@ -20,19 +20,6 @@ export interface MTaskSheetProps {
   onClose: () => void
 }
 
-/** Payload shape covering both create and update — the update schema allows
- * clearing a field with `null`, which `TodoUpdateItemRequest` itself (typed
- * against the create request) doesn't express; the store call below is cast
- * to it rather than widened to `any`. */
-interface TodoSavePayload {
-  name: string
-  description: string | null
-  due_date: string | null
-  category: string | null
-  assigned_user_id: number | null
-  priority: number
-}
-
 /**
  * Add/Edit task sheet (spec 03 §4.7 `openTask` / progress-card "New task"):
  * name, description, list (category), due date, assignee and priority.
@@ -99,7 +86,7 @@ export default function MTaskSheet({ planner, open, itemId, categories, members,
     setSaving(true)
     try {
       if (item) {
-        const payload: TodoSavePayload = {
+        const payload: TodoUpdateItemRequest = {
           name: trimmedName,
           description: description || null,
           due_date: dueDate || null,
@@ -107,7 +94,7 @@ export default function MTaskSheet({ planner, open, itemId, categories, members,
           assigned_user_id: assignedUserId,
           priority,
         }
-        await tripActions.updateTodoItem(tripId, item.id, payload as unknown as TodoUpdateItemRequest)
+        await tripActions.updateTodoItem(tripId, item.id, payload)
       } else {
         const payload: TodoCreateItemRequest = {
           name: trimmedName,