# Variable Pool Rebalance / Transfer Feature

## Summary
- Allow users to redistribute current variable pool (BudgetSession.availableCents) by setting per-category dollar targets, without changing future percent-based allocations.
- Enforce guards: sum = available, savings floor confirm, non-negative, no single category >80%, warnings when lowering savings.
- New backend endpoint performs atomic balance updates and audit logging; fixed expenses unaffected.

## API
- `POST /variable-categories/manual-rebalance`
  - Body: `{ targets: [{ id, targetCents }], forceLowerSavings?: boolean }`
  - Uses latest BudgetSession by periodStart; availableCents is the pool to balance.
  - Validations: targets cover every category; non-negative; sum(targets)=available; each ≤80% of available; lowering savings or savings <20% requires `forceLowerSavings`.
  - Transaction: update category balanceCents to targets; insert transaction(kind=`rebalance`, note snapshot); fixed plans untouched.
  - Response: `{ ok: true, availableCents, categories: [{ id, balanceCents }] }`.

## UI/UX
- New Rebalance page (Settings → Expenses tab entry point) showing availableCents and per-category balances.
- Editable dollar inputs with live total meter and inline errors for rule violations.
- Savings floor warning/confirm; optional helper to adjust one category and auto-scale others respecting floors.
- Confirmation modal summarizing before/after deltas.

## Data / Logic
- Active session = latest BudgetSession for user.
- Rebalance acts on current variable pool only; future income remains percent-based.
- Savings floor default 20% of available; lowering requires confirmation flag.

## Tests
- Sum=available happy path; savings unchanged.
- Lowering savings w/out flag → 400; with flag → OK.
- Savings total <20% w/out flag → 400; with flag → OK.
- >80% single category → 400.
- Sum mismatch → 400; negative target → 400.
- Negative existing balance allowed only if target >=0 (ending non-negative).
- Adjust-one helper unit: scaling respects floors.
- Audit entry created; fixed plans and percents unchanged.

## Assumptions
- availableCents equals dashboard “Available” variable pool.
- No localization requirements for new errors.