fix: use local variable for position to avoid cross-view contamination

When creating a task, calculateNewPositionForTask is called in a loop
for each view. Previously it wrote the calculated position back to the
shared *Task struct (t.Position), causing subsequent views to skip
their own position calculation and reuse the first view's value.
This resulted in new tasks appearing in the middle of other views
instead of at the top.

Use a local variable instead of mutating t.Position so each view
calculates its position independently.

Ref: https://github.com/go-vikunja/vikunja/issues/2329

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

pkg/models/task_position.go

@@ -303,7 +303,8 @@ func recalculateTaskPositionsForRepair(s *xorm.Session, view *ProjectView) error
 }
 
 func calculateNewPositionForTask(s *xorm.Session, a web.Auth, t *Task, view *ProjectView) (*TaskPosition, error) {
-	if t.Position == 0 {
+	position := t.Position
+	if position == 0 {
 		lowestPosition := &TaskPosition{}
 		exists, err := s.Where("project_view_id = ?", view.ID).
 			OrderBy("position asc").
@@ -327,14 +328,14 @@ func calculateNewPositionForTask(s *xorm.Session, a web.Auth, t *Task, view *Pro
 				}
 			}
 
-			t.Position = lowestPosition.Position / 2
+			position = lowestPosition.Position / 2
 		}
 	}
 
 	return &TaskPosition{
 		TaskID:        t.ID,
 		ProjectViewID: view.ID,
-		Position:      calculateDefaultPosition(t.Index, t.Position),
+		Position:      calculateDefaultPosition(t.Index, position),
 	}, nil
 }