## What
Expand/collapse child categories inline in three places:
- Dashboard → **Top spending categories**
- Cashflow → **Income sources**
- Cashflow → **Expense categories**
## How it looks
<img width="1225" height="928" alt="image"
src="https://github.com/user-attachments/assets/aadce904-bfdd-46eb-b919-8695577845d7"
/>
## Implementation
**Frontend**
- `useExpandableCategories` hook: tracks expanded set, lazy-fetches
children once, caches, resets on period change.
- `AnimatedCollapse` component for the height animation.
- Recursive rows in `BreakdownCard` and `TopCategoriesCard`.
**Backend**
- Extracted `rollUpByTree`/`displayNodeFor` out of
`CashflowAnalyticsController` into `CategoryTree::rollUp` (now shared by
cashflow + dashboard).
- Dashboard `top-categories` emits `has_children`/`is_direct` and
accepts `?parent` to drill into a category's children (children + a
direct "Parent" node), mirroring the existing cashflow breakdown drill.
- Added Spanish translations for the new toggle labels.
## Tests
- Backend: `has_children` true/false + parent-drill split (children +
direct node) for dashboard top categories.
- Component: chevron expands, lazily fetches `parent=…`, renders the
child, flips to "Hide subcategories".
- Full suite green; pint / eslint / prettier clean.
> Note: children fetch lazily on first expand (same pattern as the
Sankey inline expand).
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