usage cache is not invalidated on account/login switch (stale until TTL) #459

fetchUsageData's cache (~/.cache/ccstatusline/usage.json) is keyed only by age (CACHE_MAX_AGE 180s), not by token. After a logout/login to a different account, renders serve the prior account's usage for up to 180s. Most visible to/from Enterprise (null windows).

Invalidate the cache on token change.