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/plugins.go

@@ -0,0 +1,55 @@
+package main
+
+import "fmt"
+
+// Plugin defines a simple notification interface.
+type Plugin interface {
+    Notify(event string, payload interface{})
+}
+
+// TelegramNotifier is a mock notifier that would send a Telegram message in a real deployment.
+type TelegramNotifier struct{}
+
+func (t *TelegramNotifier) Notify(event string, payload interface{}) {
+    fmt.Printf("[TelegramNotifier] event=%s payload=%v\n", event, payload)
+}
+
+// SlackNotifier is a mock notifier that would send a Slack message in a real deployment.
+type SlackNotifier struct{}
+
+func (s *SlackNotifier) Notify(event string, payload interface{}) {
+    fmt.Printf("[SlackNotifier] event=%s payload=%v\n", event, payload)
+}
+
+// TrafficLogger logs traffic-related events for debugging/observability.
+type TrafficLogger struct{}
+
+func (l *TrafficLogger) Notify(event string, payload interface{}) {
+    fmt.Printf("[TrafficLogger] event=%s payload=%v\n", event, payload)
+}
+
+// PluginManager loads and triggers plugins.
+type PluginManager struct {
+    plugins []Plugin
+}
+
+// NewPluginManager creates a new PluginManager instance.
+func NewPluginManager() *PluginManager {
+    return &PluginManager{plugins: []Plugin{}}
+}
+
+// LoadPlugins initializes the built-in example plugins.
+func (pm *PluginManager) LoadPlugins() {
+    pm.plugins = []Plugin{
+        &TelegramNotifier{},
+        &SlackNotifier{},
+        &TrafficLogger{},
+    }
+}
+
+// Trigger dispatches an event to all registered plugins.
+func (pm *PluginManager) Trigger(event string, payload interface{}) {
+    for _, p := range pm.plugins {
+        p.Notify(event, payload)
+    }
+}