api: return empty [] instead of nil systems/channels in /api/v1/scanner

[trgosk]

The web SPA's Systems view crashes with "can't access property 'find', s.systems is null" when the scanner has no hunted systems wired (e.g. cc_hunt disabled, or running a conventional-only setup). The cause is Go's JSON marshalling: a nil []SystemHuntStatusDTO serialises to "null", but the SPA expects "[]" and calls .find()/.map() directly.

Same nil/[] mismatch existed for ConvScannerStatusDTO.Channels when no conventional channels are configured.

Normalise both slices to empty arrays in handleScannerStatus before writing the response. The "scanner == nil" early-return path now also initialises both slices for the same reason.

Verified on a running daemon with cc_hunt disabled and no conventional channels: GET /api/v1/scanner now returns
{"scan_mode":"all","systems":[],"conventional":{...,"channels":[]},...}
and the SPA renders the empty Systems view instead of throwing.

Summary

Changes

Test plan

• make vet test is green locally
• make integration is green locally (if the change touches the
  daemon)
• Added or updated tests where appropriate
• Smoke-tested against real hardware (if SDR / USB / vocoder code
  changed) — describe the dongle / capture used

Breaking changes

Docs / CHANGELOG

• Added a ## [Unreleased] bullet to CHANGELOG.md
  if this is user-visible
• Updated README.md or docs/ if
  operator-facing behaviour changed

Linked issues