Add-contribution modal previously offered zero feedback on what the
typed amount would do to goal progress. Now renders "Currently X%
saved (Y of Z)." at rest and updates on each keystroke to
"Will bring you to X% saved (Y of Z)." or "Will reach your Z target."
when the contribution would close the gap.
- New goal_contribution_preview_controller.js consumes current balance
+ target + currency + three localized template strings as Stimulus
values. Intl.NumberFormat for currency formatting (locale-correct
out of the box; fallback to currencyValue prefix on environments
that don't support it).
- ERB form-level data-controller wires the values; amount input uses
amount_data: to thread the Stimulus target / action through the
money_field helper.
- Locale: goal_contributions.new.preview_{zero,nonzero,reached} with
{percent}, {current}, {newTotal}, {target} placeholders.
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