Fix escrow amounts without bigint precision loss

apps/web/package.json

@@ -8,7 +8,7 @@
     "dev": "vite",
     "build": "vite build",
     "preview": "vite preview",
-    "test": "node --import tsx --test src/demo/trace-health-check.test.ts src/lib/config.test.ts src/dashboard/fixture-health-check.test.ts src/lib/round-status.test.ts src/components/dashboard/RoundStatusCard.test.tsx src/hooks/useDashboardData.test.ts",
+    "test": "node --import tsx --test src/demo/trace-health-check.test.ts src/lib/config.test.ts src/lib/chain.test.ts src/dashboard/fixture-health-check.test.ts src/lib/round-status.test.ts src/components/dashboard/RoundStatusCard.test.tsx src/hooks/useDashboardData.test.ts",
     "typecheck": "tsc --noEmit -p tsconfig.json"
   },
   "dependencies": {

apps/web/src/lib/amount.ts

@@ -0,0 +1,15 @@
+// Copyright (c) 2026 Sub Rosa contributors
+
+export function formatEscrowAmount(value: bigint, tokenLabel: string): string {
+  const SCALE = 10_000_000n;
+  const QUANTA = 10_000n;
+
+  const whole = value / SCALE;
+  const remainder = value % SCALE;
+  const fractional = (remainder * QUANTA + (SCALE / 2n)) / SCALE;
+
+  const normalizedWhole = fractional >= QUANTA ? whole + 1n : whole;
+  const normalizedFraction = fractional >= QUANTA ? 0n : fractional;
+
+  return `${normalizedWhole}.${normalizedFraction.toString().padStart(4, "0")} ${tokenLabel}`;
+}

apps/web/src/lib/chain.test.ts

@@ -0,0 +1,15 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import { formatEscrowAmount } from "./amount";
+
+test("formatEscrowAmount preserves exact 4-decimal display above Number.MAX_SAFE_INTEGER", () => {
+  assert.equal(formatEscrowAmount(9007199254740992n, "token"), "900719925.4741 token");
+  assert.equal(formatEscrowAmount(9007199254741500n, "token"), "900719925.4742 token");
+});
+
+test("formatEscrowAmount keeps four-decimal display for standard escrow values", () => {
+  assert.equal(formatEscrowAmount(10_000_000n, "token"), "1.0000 token");
+  assert.equal(formatEscrowAmount(1_000_000n, "token"), "0.1000 token");
+  assert.equal(formatEscrowAmount(123_456_789n, "token"), "12.3457 token");
+});

apps/web/src/lib/chain.ts

@@ -8,6 +8,8 @@ import {
 import { RoundContract } from "@sub-rosa/sdk";
 import { useMemo } from "react";
 
+import { formatEscrowAmount } from "./amount";
+
 export const LOGO_SRC = "/sub-rosa-logo.png";
 export const RPC_URL = import.meta.env.VITE_RPC_URL ?? "https://soroban-testnet.stellar.org";
 export const NETWORK =
@@ -70,7 +72,7 @@ export function toDemoEscrowAmount(value: number): bigint {
 }
 
 export function formatDemoAmount(value: bigint): string {
-  return `${(Number(value) / 10_000_000).toFixed(4)} ${ESCROW_TOKEN_LABEL}`;
+  return formatEscrowAmount(value, ESCROW_TOKEN_LABEL);
 }
 
 export async function sha256Bytes(text: string): Promise<Uint8Array> {