feat: Screentime / Time Limit read path

[ultimaterex]

Summary

• Parse init.screentimeRules into Timelimit DTO fields (quota, day-window offset, prefixed mac/tag scope)
• Rules Timelimit UI: show fields when present; honest empty state; Add Rule Time Limit → Coming soon (cap off)
• Hermetic synthetic screentime fixture (no live lab dependency)

Test plan

• go test ./internal/fwapp/ ./internal/api/ -count=1
• cd ui && npm run build
• Paired UI: Timelimit empty or list; Time Limit create shows Coming soon; no network write
• Confirm pause/delete on non-screentime rules still work