test: cover ICO binary frames and icon-source heuristics

- ico.test.ts: validate buildIcoFromPngBuffers header layout, the
  size>=256 sentinel, multi-frame offset arithmetic, and empty input;
  round-trip writeIcoWithPreferredSize to check preferred size
  ordering plus failure paths for missing source and malformed header
- icon-source.test.ts: exercise isLikelyLocalHostname across the
  localhost literal, IPv4/IPv6 forms, dotless hosts, and each suffix
  in LOCAL_HOST_SUFFIXES; cover generateDashboardIconSlugs on blank,
  single-word, and multi-word inputs; spot-check shouldPreferDashboardIcons
  for unparseable URLs, root-domain brands, and product subdomains