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.

internal/tui/oauth_tab.go

@@ -23,7 +23,6 @@ var oauthProviders = []oauthProvider{
 	{"Claude (Anthropic)", "anthropic-auth-url", "🟧"},
 	{"Codex (OpenAI)", "codex-auth-url", "🟩"},
 	{"Antigravity", "antigravity-auth-url", "🟪"},
-	{"Qwen", "qwen-auth-url", "🟨"},
 	{"Kimi", "kimi-auth-url", "🟫"},
 	{"IFlow", "iflow-auth-url", "⬜"},
 }
@@ -280,8 +279,6 @@ func (m oauthTabModel) submitCallback(callbackURL string) tea.Cmd {
 					providerKey = "codex"
 				case "antigravity-auth-url":
 					providerKey = "antigravity"
-				case "qwen-auth-url":
-					providerKey = "qwen"
 				case "kimi-auth-url":
 					providerKey = "kimi"
 				case "iflow-auth-url":