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/addresses.go

@@ -12,20 +12,20 @@ import (
 // handleGetAddress handles GET /api/v1/addresses/{chain_id}/{address}
 func (s *Server) handleGetAddress(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
-		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		writeMethodNotAllowed(w)
 		return
 	}
 
 	// Parse address from URL
 	address := r.URL.Query().Get("address")
 	if address == "" {
-		http.Error(w, "Address required", http.StatusBadRequest)
+		writeValidationError(w, fmt.Errorf("address required"))
 		return
 	}
 
 	// Validate address format
 	if !isValidAddress(address) {
-		http.Error(w, "Invalid address format", http.StatusBadRequest)
+		writeValidationError(w, ErrInvalidAddress)
 		return
 	}
 
@@ -40,7 +40,7 @@ func (s *Server) handleGetAddress(w http.ResponseWriter, r *http.Request) {
 		s.chainID, address,
 	).Scan(&txCount)
 	if err != nil {
-		http.Error(w, fmt.Sprintf("Database error: %v", err), http.StatusInternalServerError)
+		writeInternalError(w, "Database error")
 		return
 	}