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/etherscan.go

@@ -13,7 +13,10 @@ import (
 // This provides Etherscan-compatible API endpoints
 func (s *Server) handleEtherscanAPI(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
-		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		writeMethodNotAllowed(w)
+		return
+	}
+	if !s.requireDB(w) {
 		return
 	}