refactor: unify OAuth providers with i18n support

- Introduce Provider interface pattern for standard OAuth protocols
- Create unified controller/oauth.go with common OAuth logic
- Add OAuthError type for translatable error messages
- Add i18n keys and translations (zh/en) for OAuth messages
- Use common.ApiErrorI18n/ApiSuccessI18n for consistent responses
- Preserve backward compatibility for existing routes and data

oauth/registry.go

@@ -0,0 +1,43 @@
+package oauth
+
+import (
+	"sync"
+)
+
+var (
+	providers = make(map[string]Provider)
+	mu        sync.RWMutex
+)
+
+// Register registers an OAuth provider with the given name
+func Register(name string, provider Provider) {
+	mu.Lock()
+	defer mu.Unlock()
+	providers[name] = provider
+}
+
+// GetProvider returns the OAuth provider for the given name
+func GetProvider(name string) Provider {
+	mu.RLock()
+	defer mu.RUnlock()
+	return providers[name]
+}
+
+// GetAllProviders returns all registered OAuth providers
+func GetAllProviders() map[string]Provider {
+	mu.RLock()
+	defer mu.RUnlock()
+	result := make(map[string]Provider, len(providers))
+	for k, v := range providers {
+		result[k] = v
+	}
+	return result
+}
+
+// IsProviderRegistered checks if a provider is registered
+func IsProviderRegistered(name string) bool {
+	mu.RLock()
+	defer mu.RUnlock()
+	_, ok := providers[name]
+	return ok
+}