fix: API JSON error responses + navbar with dropdowns

- Add backend/libs/go-http-errors for consistent JSON errors
- REST API: use writeMethodNotAllowed, writeNotFound, writeInternalError
- middleware, gateway, search: use httperrors.WriteJSON
- SPA: navbar with Explore/Tools/More dropdowns, initNavDropdowns()
- Next.js: Navbar component with dropdowns + mobile menu

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/api/search/search.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/elastic/go-elasticsearch/v8"
 	"github.com/elastic/go-elasticsearch/v8/esapi"
+	httperrors "github.com/explorer/backend/libs/go-http-errors"
 )
 
 // SearchService handles unified search
@@ -131,13 +132,13 @@ func (s *SearchService) parseResult(source map[string]interface{}) SearchResult
 // HandleSearch handles HTTP search requests
 func (s *SearchService) HandleSearch(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
-		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		httperrors.WriteJSON(w, http.StatusMethodNotAllowed, "METHOD_NOT_ALLOWED", "Method not allowed")
 		return
 	}
 
 	query := r.URL.Query().Get("q")
 	if query == "" {
-		http.Error(w, "Query parameter 'q' is required", http.StatusBadRequest)
+		httperrors.WriteJSON(w, http.StatusBadRequest, "BAD_REQUEST", "Query parameter 'q' is required")
 		return
 	}
 
@@ -157,7 +158,7 @@ func (s *SearchService) HandleSearch(w http.ResponseWriter, r *http.Request) {
 
 	results, err := s.Search(r.Context(), query, chainID, limit)
 	if err != nil {
-		http.Error(w, fmt.Sprintf("Search failed: %v", err), http.StatusInternalServerError)
+		httperrors.WriteJSON(w, http.StatusInternalServerError, "INTERNAL_ERROR", "Search failed")
 		return
 	}