fix: resolve critical bugs in settings and import flow

Fixed two critical issues:

1. **Blocking Issue - restart_app return type error**
   - Fixed compilation error where restart_app() didn't return a value
   - Used async spawn with 100ms delay to allow response before restart
   - Prevents "unreachable code" compiler error

2. **High Priority - Import SQL doesn't refresh AppSettings cache**
   - Added reload_settings() function to refresh in-memory settings cache
   - Integrated into import flow to ensure imported settings take effect
   - Prevents imported settings being overwritten by stale memory cache
   - Affects: language, config directories, auto-launch, custom endpoints

Changes:
- src/commands/settings.rs: Async delayed restart with proper return value
- src/settings.rs: New reload_settings() to sync memory cache from DB
- src/commands/import_export.rs: Call reload_settings() after SQL import

Verified: cargo clippy --lib and pnpm typecheck both pass

src-tauri/src/settings.rs

@@ -260,6 +260,15 @@ pub fn update_settings(mut new_settings: AppSettings) -> Result<(), AppError> {
     Ok(())
 }
 
+/// 从数据库重新加载设置到内存缓存
+/// 用于导入配置等场景，确保内存缓存与数据库同步
+pub fn reload_settings() -> Result<(), AppError> {
+    let fresh_settings = load_initial_settings();
+    let mut guard = settings_store().write().expect("写入设置锁失败");
+    *guard = fresh_settings;
+    Ok(())
+}
+
 pub fn ensure_security_auth_selected_type(selected_type: &str) -> Result<(), AppError> {
     let mut settings = get_settings();
     let current = settings