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/middleware/security.go

@@ -3,6 +3,8 @@ package middleware
 import (
 	"net/http"
 	"strings"
+
+	httperrors "github.com/explorer/backend/libs/go-http-errors"
 )
 
 // SecurityMiddleware adds security headers
@@ -52,7 +54,7 @@ func (m *SecurityMiddleware) BlockWriteCalls(next http.Handler) http.Handler {
 			if !strings.Contains(path, "weth") && !strings.Contains(path, "wrap") && !strings.Contains(path, "unwrap") {
 				// Block other write operations for Track 1
 				if strings.HasPrefix(path, "/api/v1/track1") {
-					http.Error(w, "Write operations not allowed for Track 1 (public)", http.StatusForbidden)
+					httperrors.WriteJSON(w, http.StatusForbidden, "FORBIDDEN", "Write operations not allowed for Track 1 (public)")
 					return
 				}
 			}