fix(proxy): harden takeover-residue recovery across config-dir switches (#4076)

Changing app_config_dir relocates the SQLite database, so a restart
triggered while proxy takeover is active used to strand the live
configs: the new instance reads a fresh DB with no live backups, the
first-run import then persisted the PROXY_MANAGED placeholder as the
`default` provider, and the no-backup recovery path wrote that
placeholder right back to the live files — leaving Claude/Codex/Gemini
pointed at a dead local proxy with no automatic way out.

Three orthogonal fixes, defense in depth:

- restart_app now awaits cleanup_before_exit() before app.restart().
  Since #4069 the ExitRequested handler intentionally defers restart
  requests to Tauri's default re-exec without custom cleanup, which is
  correct for same-DB restarts but not for this command's dir-change
  use case: only the old instance holds the backups needed to restore
  the taken-over live files, so it must restore them while its event
  loop is still alive.
- import_default_config refuses to import a live config that is under
  proxy takeover (placeholder detected), instead of persisting it as
  the current provider.
- restore_live_from_ssot_for_app validates that the current provider's
  settings_config does not itself contain takeover placeholders before
  writing it back; polluted SSOT now falls through to the placeholder
  cleanup fallback.

Regression tests cover the import guard and the no-backup recovery
path (the latter fails before this change by writing PROXY_MANAGED
back to live).

src-tauri/src/commands/settings.rs

@@ -80,6 +80,12 @@ pub async fn restart_app(app: AppHandle) -> Result<bool, String> {
     // 在后台延迟重启，让函数有时间返回响应
     tauri::async_runtime::spawn(async move {
         tokio::time::sleep(tokio::time::Duration::from_millis(100)).await;
+        // app.restart() 走 RESTART_EXIT_CODE 路径，ExitRequested 处理器会直接
+        // 放行给 Tauri 默认 re-exec，不执行代理/Live 清理。但本命令用于
+        // app_config_dir 变更后的重启：新实例会切到新数据库，拿不到旧库里的
+        // Live 备份，无法恢复被接管的 Live 配置。因此必须趁旧实例的事件循环
+        // 仍存活，在这里同步完成恢复（保留代理状态，新实例启动时自动重新接管）。
+        crate::cleanup_before_exit(&app).await;
         app.restart();
     });
     Ok(true)