[P3] detail.js refetches the entire scan (size=10000) on every prev/next

[eboudrant]

Priority: P3 · Review finding: #20

static/js/detail.js _detailNext/_detailPrev use navigateReplace, which re-runs showDetail → _loadDetail → apiGet(/api/scans/:id?page=0&size=10000). Every arrow-key press re-downloads and re-parses the full scan (potentially MBs on large scans).

Fix: cache _detailFailures keyed by scanId and skip the refetch when only the filename changed (refresh on scan change or via the watch poll).