fix(frontend): show exact Sats for swap order amounts

[21Mill]

Problem

Swap orders (currency code 1000) are denominated in Sats, but the order amount in OrderDetails was rendered through amountToString(), which rounds to 4 significant figures (toPrecision(4)).

For a real order with amount = 0.01012540 BTC, the interface showed 1,013,000 Sats, while the "Confirm sent" button — which uses the exact order.amount via toFixed(8) — showed 0.01012540 BTC = 1,012,540 Sats.

The two figures disagree. The peer expects exactly order.amount (1,012,540 Sats), so the rounded value in the interface is misleading and could cause the payer to send the wrong amount.

Root cause

toPrecision(4) is the right rounding for fiat amounts, but when the swap amount is expressed in Sats (amount * 1e8), it collapses 1012540 to 1.013e6 → 1,013,000.

Fix

• Add a dedicated btcToSatsString() helper in prettyNumbers.ts that formats the exact integer Sat amount (with range support), using Math.round(btc * 1e8) instead of significant-figure rounding.
• Use it in OrderDetails for the currency === 1000 branch, so the displayed Sats always match the exact contract amount (and the "Confirm sent" button).
• The intentionally compact significant-figure rounding in the book table / chart tooltip / maker preview is left untouched.

Tests

Adds unit tests for btcToSatsString, including a regression test: 0.01012540 BTC → "1,012,540" (previously 1,013,000).

Verification

Confirmed against a real order from the backend (amount: "0.01012540", currency: 1000, payment_method: "On-Chain BTC"): the expected amount is 1,012,540 Sats, which the fix now renders correctly.

Checklist before merging

• Install pre-commit and initialize it: pip install pre-commit, then pre-commit install. Pre-commit installs git hooks that automatically check the codebase. If pre-commit fails when you commit your changes, please fix the problems it points out.