## Summary
Adds nested categories (parent → child, up to **3 levels**) across the
app. Children inherit their parent's type and cashflow direction, and
every category selector now renders the hierarchy as an indented tree.
Gated behind the `CategoryTree` Pennant flag (off by default).
## Backend
- **Migration**: nullable self-referencing `parent_id`; uniqueness
scoped per-parent via a `parent_unique_marker` virtual column (root
names stay unique). New composite unique created before dropping the old
one so the `user_id` FK keeps a supporting index.
- **`CategoryTree` service**: descendant/ancestor resolution, depth &
cycle checks, type cascade, subtree deletion.
- **Validation**: depth limit, cycle prevention, inherited + locked
child type.
- **Delete strategies**: reparent (default), promote to root, or cascade
(uncategorizes affected transactions).
- **Transaction filter** expands a selected parent to its descendants.
- **Cashflow Sankey & breakdown** roll up to top-level parents with
click-to-drill (children + a parent "direct" node).
- **Budgets** tracking a parent also count their children's
transactions.
- **Unified** the frontend category query behind
`Category::forDisplay()` / `FRONTEND_COLUMNS` (8 call sites) so every
selector receives the full Category shape, including `parent_id`.
## Frontend
- New `category-tree.ts` helpers (build/flatten/descendants/path,
tree-aware selection toggle + tri-state).
- **Settings page**: indented tree, sortable by name/color/type
(siblings sorted, hierarchy preserved), parent picker, delete-strategy
dialog.
- **Combobox** (transaction table cell + edit/create modal + parent
picker): indented tree, search keeps matches with their ancestors.
- **Transaction filter**: indented tree, tri-state cascading selection
(parent ↔ children), themed checkboxes, selected branches float to top
on open, ancestor-aware search.
- **Categorizer palette** and **budget multi-select**: same indented
tree + ancestor-aware search.
- **Sankey**: click a parent node to drill into its children, with
breadcrumb.
- Pennant `CategoryTree` flag gates the parent UI.
## Tests
- Pest: model/validation, delete strategies, filter expansion, cashflow
rollup/drill, budget child inclusion.
- Vitest: tree-aware selection logic.
- All green; Pint / ESLint / Prettier clean.
## Rollout
Flag is off by default — enable per user with:
\`\`\`
php artisan feature:enable "App\\Features\\CategoryTree" you@example.com
\`\`\`
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