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/gateway/gateway.go

@@ -6,6 +6,8 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+
+	httperrors "github.com/explorer/backend/libs/go-http-errors"
 )
 
 // Gateway represents the API gateway
@@ -51,13 +53,13 @@ func (g *Gateway) handleRequest(proxy *httputil.ReverseProxy) http.HandlerFunc {
 
 		// Authentication
 		if !g.auth.Authenticate(r) {
-			http.Error(w, "Unauthorized", http.StatusUnauthorized)
+			httperrors.WriteJSON(w, http.StatusUnauthorized, "UNAUTHORIZED", "Unauthorized")
 			return
 		}
 
 		// Rate limiting
 		if !g.rateLimiter.Allow(r) {
-			http.Error(w, "Rate limit exceeded", http.StatusTooManyRequests)
+			httperrors.WriteJSON(w, http.StatusTooManyRequests, "RATE_LIMIT_EXCEEDED", "Rate limit exceeded")
 			return
 		}