feat(app): add WGRplane web UI and backend features

- Add Vue 3 frontend with glassmorphism design (Tailwind CSS)
- Add Go backend handlers: auth, webhooks, stats, scheduler, validation
- Add i18n support (EN, ID, ZH)
- Add Swagger docs and API handlers
- Add nftables integration and plugins support
- Remove deprecated go.mod (migrated to wgrplane)

app/i18n.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+  "encoding/json"
+  "log"
+  "path/filepath"
+  "github.com/nicksnyder/go-i18n/v2/i18n"
+  "golang.org/x/text/language"
+)
+
+var bundle *i18n.Bundle
+
+func init() {
+  if err := initI18nBundle(); err != nil {
+    // Fail fast during startup in tests/builds
+    log.Fatalf("failed to initialize i18n bundle: %v", err)
+  }
+}
+
+func initI18nBundle() error {
+  // Initialize the i18n bundle with English as default
+  bundle = i18n.NewBundle(language.English)
+  bundle.RegisterUnmarshalFunc("json", json.Unmarshal)
+
+  // Load translation files from app/ directory
+  files := []string{
+    filepath.Join("app", "active.en.json"),
+    filepath.Join("app", "active.id.json"),
+    filepath.Join("app", "active.zh.json"),
+  }
+  for _, f := range files {
+    if _, err := bundle.LoadMessageFile(f); err != nil {
+      return err
+    }
+  }
+  return nil
+}
+
+// Translate returns the translated string for a given locale and message ID.
+func Translate(locale, messageID string) string {
+  loc := i18n.NewLocalizer(bundle, locale)
+  s, err := loc.Localize(&i18n.LocalizeConfig{MessageID: messageID})
+  if err != nil || s == "" {
+    // Fallback to the messageID if translation is missing
+    return messageID
+  }
+  return s
+}