refactor: harden diagnostics and consolidate runtime code

audit/internal/webui/api_services_system.go

@@ -99,7 +99,10 @@ func (h *handler) handleAPINetworkDHCP(w http.ResponseWriter, r *http.Request) {
 	var req struct {
 		Interface string `json:"interface"`
 	}
-	_ = json.NewDecoder(r.Body).Decode(&req)
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
 
 	result, err := h.applyPendingNetworkChange(func() (app.ActionResult, error) {
 		if req.Interface == "" || req.Interface == "all" {
@@ -167,19 +170,9 @@ func (h *handler) handleAPIExportList(w http.ResponseWriter, r *http.Request) {
 }
 
 func (h *handler) handleAPIExportUSBTargets(w http.ResponseWriter, _ *http.Request) {
-	if h.opts.App == nil {
-		writeError(w, http.StatusServiceUnavailable, "app not configured")
-		return
-	}
-	targets, err := h.opts.App.ListRemovableTargets()
-	if err != nil {
-		writeError(w, http.StatusInternalServerError, err.Error())
-		return
-	}
-	if targets == nil {
-		targets = []platform.RemovableTarget{}
-	}
-	writeJSON(w, targets)
+	writeAPIListResponse(w, h.opts.App != nil, func() ([]platform.RemovableTarget, error) {
+		return h.opts.App.ListRemovableTargets()
+	})
 }
 
 func (h *handler) handleAPIBlackboxStatus(w http.ResponseWriter, _ *http.Request) {