## What
Clicking **Analysis** on the transactions page opens a chart of spending
by category. Two improvements:
### Sub-category breakdown
Previously every expense rolled up to its top-level category. Now each
parent shows its total with the sub-categories that carry spending
nested beneath it.
- New `CategoryTree::spendingBreakdown()` builds a two-level tree: each
root with its rolled-up total + immediate sub-categories. Grand-children
fold into their level-2 ancestor. Spend booked directly on a parent that
is *also* split across sub-categories surfaces as a **Direct** child, so
the children always sum to the parent total.
- `by_category` slices now carry a `children[]` array.
- The legend renders each parent row, then indented sub-category rows
(name, % of parent, amount). The donut stays at parent level.
### Clearer colors
Monochrome schemes (blue, pink, neutral) run darkest→lightest, so
adjacent slices were nearly identical. A new `alternateContrast()` zips
the two palette halves (`chart-1,5,2,6,3,7,4,8`) so neighbouring slices
alternate between dark and light ends. Sub-category dots reuse the
parent color at reduced opacity.
## Tests
Added coverage for sub-category nesting, the Direct child, grand-child
folding, and the flat (no-children) case. All analysis + CategoryTree
tests pass.
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