Control rounding factor value in Create offer screen

[peterzen]

Add "Round amount to" privacy control on the Create Offer screen

Bisq already rounds every fiat trade to whole units (factor = 1), but currently there is no way to control that value, often resulting in amounts such as 301, 302 etc with no way to override. From a privacy PoV, it would be very useful to be able to control the already-existing factor parameter as an opt-in, coarser, per-offer choice in order.

Suggestion

Add a small "Round amount to" dropdown to the maker's Create Offer screen that lets the user round the Amount in <fiat> to receive to a coarser, common-looking figure (e.g. 302 → 300) for privacy. The control sits next to the existing privacy icon in that field and reuses Bisq's existing amount-rounding machinery.

Related issue: bisq-network/bisq#1511

Mock-up

Placement & pattern

• Location: inside the "Amount in <fiat> to receive" field, immediately to the right of the existing privacy icon. No changes to the Advanced-options layout.

• UI pattern: Option A — a single icon MenuButton with a chevron, spreadsheet-toolbar style. Its face doubles as a status indicator of the active granularity (e.g. ·00 ▾).

• Rounding mode is nearest (matches existing Math.round behavior), not separate up/down.

• Currency-aware option set: EUR/USD/GBP-style currencies use 1 / 5 / 10 / 50 / 100; large-magnitude currencies (e.g. JPY/KRW) use a scaled set (100 / 500 / 1000 / 5000). Default to the EUR-style list.

Behavior

• Default = "No rounding" = factor 1, i.e. current behavior; the receive amount is left as it is calculated today.
• Per-offer only. The selection lives in the offer view-model/data-model and is not persisted to Preferences; it resets to factor 1 each time Create Offer is opened.
• Maker side only. No change to the Take Offer screen in this iteration.
• Rounding adjusts the actual amount (both the BTC amount and the fiat volume move together), so the rounded figure is what is really transferred — consistent with Option to have fiat values rounded to full euros, 0 cents #1511.
• Factors unreachable for the current amount/price (e.g. would breach the trade limit or fall below the minimum) are disabled in the menu (keep menu layout stable)
• HAL Cash: already forces factor 10; the control is overridden/disabled for that payment method.
• Altcoin/BSQ offers: control hidden (fiat-only)
• Min amount: the same factor is applied to the Minimum amount field so range offers stay self-consistent.
• Range offers: the maker's chosen factor rounds the offer endpoints; the taker re-rounds their chosen amount at factor 1 on their side. Surface this in the control's tooltip.