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/provider.go

@@ -0,0 +1,36 @@
+package oauth
+
+import (
+	"context"
+
+	"github.com/QuantumNous/new-api/model"
+	"github.com/gin-gonic/gin"
+)
+
+// Provider defines the interface for OAuth providers
+type Provider interface {
+	// GetName returns the display name of the provider (e.g., "GitHub", "Discord")
+	GetName() string
+
+	// IsEnabled returns whether this OAuth provider is enabled
+	IsEnabled() bool
+
+	// ExchangeToken exchanges the authorization code for an access token
+	// The gin.Context is passed for providers that need request info (e.g., for redirect_uri)
+	ExchangeToken(ctx context.Context, code string, c *gin.Context) (*OAuthToken, error)
+
+	// GetUserInfo retrieves user information using the access token
+	GetUserInfo(ctx context.Context, token *OAuthToken) (*OAuthUser, error)
+
+	// IsUserIDTaken checks if the provider user ID is already associated with an account
+	IsUserIDTaken(providerUserID string) bool
+
+	// FillUserByProviderID fills the user model by provider user ID
+	FillUserByProviderID(user *model.User, providerUserID string) error
+
+	// SetProviderUserID sets the provider user ID on the user model
+	SetProviderUserID(user *model.User, providerUserID string)
+
+	// GetProviderPrefix returns the prefix for auto-generated usernames (e.g., "github_")
+	GetProviderPrefix() string
+}