perf(usage-store): Math.max(...tsValues) spread overflows V8 argument stack on large record sets

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Problem

apps/desktop/src/main/usage/store.ts L153–154

const tsValues = records.map((r) => r.ts); // intermediate array
const spanMs = tsValues.length > 1
  ? Math.max(...tsValues) - Math.min(...tsValues) // ← spread
  : 0;

Math.max(...array) passes array elements as function arguments via spread. V8's function call argument limit is ~65,536. When records exceeds that count, a RangeError: Maximum call stack size exceeded is thrown.

Suggested fix

Replace the spread with a reduce or for loop:

let minTs = Infinity, maxTs = -Infinity;
for (const r of records) {
  if (r.ts < minTs) minTs = r.ts;
  if (r.ts > maxTs) maxTs = r.ts;
}

This also eliminates the intermediate tsValues array allocation, saving one O(n) pass.