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/rest/config.go

@@ -15,7 +15,7 @@ var dualChainTokenListJSON []byte
 func (s *Server) handleConfigNetworks(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
 		w.Header().Set("Allow", "GET")
-		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		writeMethodNotAllowed(w)
 		return
 	}
 	w.Header().Set("Content-Type", "application/json")
@@ -27,7 +27,7 @@ func (s *Server) handleConfigNetworks(w http.ResponseWriter, r *http.Request) {
 func (s *Server) handleConfigTokenList(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
 		w.Header().Set("Allow", "GET")
-		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		writeMethodNotAllowed(w)
 		return
 	}
 	w.Header().Set("Content-Type", "application/json")