fix: nil slices return [] not null in all API responses

internal/api/projects.go

@@ -27,9 +27,6 @@ func (s *Server) listProjects(w http.ResponseWriter, r *http.Request) {
 		respondError(w, http.StatusInternalServerError, "failed to list projects: "+err.Error())
 		return
 	}
-	if projects == nil {
-		projects = []store.Project{}
-	}
 	respondJSON(w, http.StatusOK, projects)
 }
 

internal/api/response.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"log/slog"
 	"net/http"
+	"reflect"
 )
 
 // envelope is the standard API response wrapper.
@@ -14,7 +15,15 @@ type envelope struct {
 }
 
 // respondJSON writes a JSON success response with the given status code and data.
+// Nil slices are converted to empty arrays to avoid "null" in JSON output.
 func respondJSON(w http.ResponseWriter, status int, data any) {
+	// Convert nil slices to empty arrays so JSON encodes as [] not null.
+	if data != nil {
+		v := reflect.ValueOf(data)
+		if v.Kind() == reflect.Slice && v.IsNil() {
+			data = reflect.MakeSlice(v.Type(), 0, 0).Interface()
+		}
+	}
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
 	if err := json.NewEncoder(w).Encode(envelope{Success: true, Data: data}); err != nil {