chore: remove Qwen support from SDK and internal components

- Deleted `QwenAuthenticator`, internal `qwen_auth`, and `qwen_executor` implementations.
- Removed all Qwen-related OAuth flows, token handling, and execution logic.
- Cleaned up dependencies and references to Qwen across the codebase.

cmd/server/main.go

@@ -61,7 +61,6 @@ func main() {
 	var codexLogin bool
 	var codexDeviceLogin bool
 	var claudeLogin bool
-	var qwenLogin bool
 	var iflowLogin bool
 	var iflowCookie bool
 	var noBrowser bool
@@ -82,7 +81,6 @@ func main() {
 	flag.BoolVar(&codexLogin, "codex-login", false, "Login to Codex using OAuth")
 	flag.BoolVar(&codexDeviceLogin, "codex-device-login", false, "Login to Codex using device code flow")
 	flag.BoolVar(&claudeLogin, "claude-login", false, "Login to Claude using OAuth")
-	flag.BoolVar(&qwenLogin, "qwen-login", false, "Login to Qwen using OAuth")
 	flag.BoolVar(&iflowLogin, "iflow-login", false, "Login to iFlow using OAuth")
 	flag.BoolVar(&iflowCookie, "iflow-cookie", false, "Login to iFlow using Cookie")
 	flag.BoolVar(&noBrowser, "no-browser", false, "Don't open browser automatically for OAuth")
@@ -484,8 +482,6 @@ func main() {
 	} else if claudeLogin {
 		// Handle Claude login
 		cmd.DoClaudeLogin(cfg, options)
-	} else if qwenLogin {
-		cmd.DoQwenLogin(cfg, options)
 	} else if iflowLogin {
 		cmd.DoIFlowLogin(cfg, options)
 	} else if iflowCookie {