fix(mcp): skip sync when target CLI app is not installed

Add guard functions to check if Claude/Codex/Gemini CLI has been
initialized before attempting to sync MCP configurations. This prevents
creating unwanted config files in directories that don't exist.

- Claude: check ~/.claude dir OR ~/.claude.json file exists
- Codex: check ~/.codex dir exists
- Gemini: check ~/.gemini dir exists

When the target app is not installed, sync operations now silently
succeed without writing any files, allowing users to manage MCP servers
for apps they actually use without side effects on others.

src-tauri/tests/import_export_sync.rs

@@ -154,6 +154,12 @@ fn sync_enabled_to_codex_writes_enabled_servers() {
     let _guard = test_mutex().lock().expect("acquire test mutex");
     reset_test_fs();
 
+    // 模拟 Codex 已安装/已初始化：存在 ~/.codex 目录
+    let path = cc_switch_lib::get_codex_config_path();
+    if let Some(parent) = path.parent() {
+        fs::create_dir_all(parent).expect("create codex dir");
+    }
+
     let mut config = MultiAppConfig::default();
     config.mcp.codex.servers.insert(
         "stdio-enabled".into(),
@@ -170,7 +176,6 @@ fn sync_enabled_to_codex_writes_enabled_servers() {
 
     cc_switch_lib::sync_enabled_to_codex(&config).expect("sync codex");
 
-    let path = cc_switch_lib::get_codex_config_path();
     assert!(path.exists(), "config.toml should be created");
     let text = fs::read_to_string(&path).expect("read config.toml");
     assert!(
@@ -594,6 +599,11 @@ command = "echo"
 fn sync_claude_enabled_mcp_projects_to_user_config() {
     let _guard = test_mutex().lock().expect("acquire test mutex");
     reset_test_fs();
+    let home = ensure_test_home();
+
+    // 模拟 Claude 已安装/已初始化：存在 ~/.claude 目录
+    fs::create_dir_all(home.join(".claude")).expect("create claude dir");
+
     let mut config = MultiAppConfig::default();
 
     config.mcp.claude.servers.insert(