## Problem
On the create/edit transaction form, the amount field uses
`inputMode="decimal"`. On iOS the numeric keypad that this triggers has
**no minus key**, so users cannot type the leading `-` the input relies
on to record a negative amount (e.g. an outgoing transfer).
## Fix
Add an opt-in `allowNegative` prop to `AmountInput` that renders a small
`+`/`−` sign-toggle button at the **start** of the field. It works with
any keyboard, so it does not depend on the OS keypad. It is enabled only
on the create-transaction amount input; budgets, balances and loan
fields keep their current UI.
- The button turns red with a `−` when the amount is negative, and shows
`+` otherwise (`aria-pressed` reflects the state).
- Toggle fires on `click` (works for both pointer and keyboard);
`onPointerDown` `preventDefault` keeps the input focused to avoid a
blur/reformat race.
- A leading `-` set via the toggle **before** typing is preserved on
focus, so the sign is not silently dropped.
- The currency symbol shifts right when the toggle is present so the two
never overlap; layout is unchanged when `allowNegative` is off.
## Refactor
Extracted the repeated `evaluateMathExpression(x) ?? parseInputValue(x)`
string→cents resolution (previously duplicated across blur, Enter, and
the new toggle) into a single `resolveCents()` helper.
## Tests
`resources/js/components/ui/amount-input.test.tsx` — the toggle is
hidden unless `allowNegative`, flips positive↔negative, and preserves
the sign when focusing after toggling an empty field.
## QA
Manually verified in the running app (create-transaction dialog): typing
an amount and toggling the sign both ways updates the value and the
button state correctly, on both prefix (USD `$`) and suffix (EUR `€`)
currency layouts.
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