feat (auth): CLI OAuth supports pasting callback URLs to complete login

- Added callback URL resolution and terminal prompt logic
  - Codex/Claude/iFlow/Antigravity/Gemini login supports callback URL or local callback completion
  - Update Gemini login option signature and manager call
  - CLI default prompt function is compatible with null input to continue waiting

sdk/auth/oauth_callback.go

@@ -0,0 +1,41 @@
+package auth
+
+import (
+	"fmt"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/misc"
+)
+
+func promptForOAuthCallback(prompt func(string) (string, error), provider string) (<-chan *misc.OAuthCallback, <-chan error) {
+	if prompt == nil {
+		return nil, nil
+	}
+
+	resultCh := make(chan *misc.OAuthCallback, 1)
+	errCh := make(chan error, 1)
+
+	go func() {
+		label := provider
+		if label == "" {
+			label = "OAuth"
+		}
+		input, err := prompt(fmt.Sprintf("Paste the %s callback URL (or press Enter to keep waiting): ", label))
+		if err != nil {
+			errCh <- err
+			return
+		}
+
+		parsed, err := misc.ParseOAuthCallback(input)
+		if err != nil {
+			errCh <- err
+			return
+		}
+		if parsed == nil {
+			return
+		}
+
+		resultCh <- parsed
+	}()
+
+	return resultCh, errCh
+}