The `subcategories` method queries `WHERE parent_id = category.id`, but
for the synthetic uncategorized budget category, `category.id` is nil.
This caused `WHERE parent_id IS NULL` to match ALL top-level categories,
making them appear as subcategories of uncategorized. This inflated
actual_spending and produced a large negative available_to_spend.
Add a nil guard on category.id to return an empty relation for synthetic
categories.
Fixes#819
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>