## Why
Some banks return transactions without a balance column. The CSV/Excel
importer happily creates the transactions but leaves the account without
any balance history, making the account look incomplete.
## What
Opt-in flow that derives per-day balances from the transactions
themselves, using a single reference point (the balance on the date of
the latest transaction).
### Mapping step
- New checkbox **"Calculate balances from transactions"** next to the
Balance column select.
- Available only when no balance column is mapped (disabled + visually
faded otherwise).
- When checked, a balance input appears labeled with the latest
transaction date using a relative format:
- `Today`
- `Yesterday`
- `Monday, 3 of Jun`
- If an `AccountBalance` already exists for that date, it is pre-filled
automatically (no need to ask) with a small hint.
- The reference balance is **mandatory** when the checkbox is checked;
the Next button is disabled until it is provided.
### Preview step
- A new `Balance` column shows the balance about to be created for each
date, regardless of whether it came from the file or was calculated.
### Persistence
- Balances are computed by walking backwards across distinct transaction
dates and subtracting each day's net movement.
- They flow through the existing import path that POSTs to
`AccountBalanceController@store`, so no new endpoint was needed.
## Feature flag
Hidden behind a new Pennant feature, **off by default**:
```bash
php artisan feature:enable CalculateBalancesOnImport user@example.com
# or
php artisan feature:enable CalculateBalancesOnImport all
```
The flag is exposed to the frontend via Inertia shared props
(`features.calculateBalancesOnImport`).
## Tests
- New vitest cases for `getLatestTransactionDate` and
`calculateBalancesFromTransactions` (sparse dates, reference date
without txns, empty list).
- `LocalizationTest` passes (new ES strings added).
- Full vitest suite green (106 tests).
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