fix(clean): report sub-megabyte Service Worker cleanups in KB

[LuisMIguelFurlanettoSousa]

Summary

• Fixes [BUG] Service Worker cleanup under 1MB is reported as "0MB" #1228. clean_service_worker_cache reported any cleanup under 1 MB as 0MB.
• It tracked the cleaned amount in KB and formatted it with a hand-rolled cleaned_size / 1024 integer division, which truncates anything below 1024 KB to 0. Service Worker CacheStorage folders are per-origin and often only a few hundred KB, so this was easy to hit.
• Switch to the shared bytes_to_human formatter — the same one every other cleaner uses — so sub-MB amounts render as KB (e.g. 922KB). MB amounts now carry the standard one-decimal format used elsewhere (e.g. 5.2MB instead of 5MB), making this row consistent with the rest of Mole's output.

Safety Review

• No. This only changes how the freed size is formatted for display; the scan, protection, whitelist, and removal logic are untouched.

Tests

• bats tests/clean_system_caches.bats — added clean_service_worker_cache reports sub-megabyte cleanups as KB, not 0MB (feeds a 900 KB cleanup and asserts the row shows KB, never 0MB). Updated the existing color test's expectation from 1MB to 1.0MB to match the shared formatter. The rest of the Service Worker tests pass unchanged.
• shellcheck lib/clean/caches.sh and shfmt -i 4 -ci -sr -d lib/clean/caches.sh are clean.

Safety-related changes

• None.