fix(web): stop formatUsageNumber rendering 1000.0K at unit boundaries

[abhay-codes07]

Problem

formatUsageNumber scales the value and formats it with one decimal:

if (value >= 1_000) {
  const thousands = value / 1_000
  return thousands % 1 === 0 ? `${thousands}K` : `${thousands.toFixed(1)}K`
}

toFixed(1) rounds, so any value from 999,950 to 999,999 becomes 999.95..999.999 → "1000.0", and the usage counter shows "1000.0K" instead of "1.0M". The same artifact appears at the million boundary (999,999,950 → "1000.0M").

Fix

When the scaled value rounds up to 1000, promote it to the next unit, and add a billions tier so very large counts read "1.0B" instead of "1000.0M":

if (value >= 1_000) {
  const thousands = value / 1_000
  return thousands >= 999.95
    ? withSuffix(value / 1_000_000, "M")
    : withSuffix(thousands, "K")
}

Tests

Added billing-utils.test.ts (bun:test) covering both boundaries: 999,950 and 999,999 → "1.0M", 999,949 → "999.9K" (still rounds within band), 999,999,950 → "1.0B", plus the existing 50K / 1.5M / 2M cases. 5 tests pass.

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