Fix: defensive frontend + panic recovery logging middleware

Frontend:
- JobDetail loadLogs: fallback to [] when API returns null
- JobDetail loadJob: pairs ?? [] guard on /api/sync-pairs 500
- JobHistory: Array.isArray guard on job list response
- api client: return undefined for null body instead of throwing

Backend:
- handlers_jobs GetLog: return [] instead of null when no log rows
- router: custom recoverer middleware that logs panics to slog
  with full stack trace, method, and path

internal/api/handlers_jobs.go

@@ -166,7 +166,11 @@ func (h *JobHandler) GetLog(w http.ResponseWriter, r *http.Request) {
 
 	count, _ := logRepo.CountByJobID(id)
 	w.Header().Set("X-Total-Count", fmt.Sprintf("%d", count))
-	writeJSON(w, logs)
+	if logs == nil {
+		writeJSON(w, []any{})
+	} else {
+		writeJSON(w, logs)
+	}
 }
 
 func (h *JobHandler) DownloadLog(w http.ResponseWriter, r *http.Request) {