## Summary
Two improvements to the Cashflow Sankey chart, plus a fix reported for
real users whose chart rendered with overlapping labels.
### 1. Fix overlapping labels on small categories
recharts spaces same-column Sankey nodes by exactly `nodePadding` and
distributes the height **proportionally to value**, so tiny categories
end up separated by only that padding regardless of canvas height. It
was 24px while a two-line label is 30px tall, so adjacent labels
overlapped (e.g. *Regalos* / *Otros* in the monthly view, *Ingreso
desconocido* / *Pagos devueltos* in the yearly view).
Set `NODE_PADDING` above the label height so two adjacent labels can
never collide, whatever the bar size. This is independent of width, so
it fixes desktop and mobile alike.
### 2. Expand income categories (drill-down)
Income categories can now be drilled down into their subcategories,
mirroring the existing expense behaviour. The `sankey` endpoint already
returns both sides for a given `parent`, so this is frontend-only:
- The open drill-down is keyed by `{id, kind}` — a category can
legitimately appear on both the income and expense side.
- Income subcategories link `child -> parent` and form a column to the
**left** of the parent (expense subcategories stay on the right).
- The chart-height budget now accounts for the combined leftmost column
(income siblings + subcategories share it), so a many-source drill-down
can't drive recharts to a negative row scale and break the whole chart.
- The expand chevron points the way the column opens: income shows `<`
collapsed / `>` expanded; expense keeps `>` collapsed / `v` expanded.
The income/expense node-building was also de-duplicated into a single
helper.
## Test plan
- `sankey-chart.test.tsx`: 7 unit tests green (added income-expansion
coverage alongside the existing expense one).
- `tsc --noEmit`, `eslint`, `prettier` clean.
- Manual browser QA (Playwright) on a real account, desktop **and**
mobile: monthly + yearly views (overlap gone), expand income, switch to
expand expense (no regression), collapse, chevron direction, responsive
horizontal scroll.
## Demo
<!-- PLACEHOLDER: drag the demo video here -->
_📎 Attach the demo video here — file generated at
`~/Downloads/sankey-cashflow-demo.mp4`_
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