feat(welcome): show first-run welcome dialog on fresh install

Introduce a one-time welcome dialog that explains CC Switch's workflow
to new users: how their existing config is preserved as a "default"
provider and how the bundled "Official" preset enables one-click revert.
Upgrade users are excluded by checking is_providers_empty() at startup
and never see the dialog.

Persistence follows the existing *_confirmed convention in AppSettings
(proxy/usage/stream_check/failover), stored in settings.json. The field
is only written when the user explicitly clicks the confirm button,
keeping its semantics strictly about user acknowledgement.

Also adds two reusable DAO helpers:
- Database::is_providers_empty for fresh-install detection, using
  EXISTS(SELECT 1) for a short-circuit query.
- Database::get_bool_flag accepting "true" | "1", with
  init_default_official_providers migrated to use it.

Dialog copy in zh/en/ja uses conditional phrasing so it stays
accurate whether or not existing live config was found.

src-tauri/src/database/dao/providers.rs

@@ -502,6 +502,20 @@ impl Database {
         }))
     }
 
+    /// 判断 providers 表是否为空（全 app_type 一起算）。
+    ///
+    /// 用于区分"全新安装"和"升级用户"：在启动流程 import/seed 之前调用。
+    /// 使用 `EXISTS` 短路查询，比 `COUNT(*)` 在将来表变大时更高效。
+    pub fn is_providers_empty(&self) -> Result<bool, AppError> {
+        let conn = lock_conn!(self.conn);
+        let exists: bool = conn
+            .query_row("SELECT EXISTS(SELECT 1 FROM providers)", [], |row| {
+                row.get(0)
+            })
+            .map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(!exists)
+    }
+
     /// 仅获取指定 app 下所有 provider 的 id 集合。
     ///
     /// 比 `get_all_providers` 轻量得多：只读 id 列、无 endpoint 子查询。
@@ -568,11 +582,11 @@ impl Database {
     pub fn init_default_official_providers(&self) -> Result<usize, AppError> {
         use crate::database::dao::providers_seed::OFFICIAL_SEEDS;
 
-        // flag 检查：已执行过则跳过
-        if let Ok(Some(flag)) = self.get_setting("official_providers_seeded") {
-            if flag == "true" || flag == "1" {
-                return Ok(0);
-            }
+        if self
+            .get_bool_flag("official_providers_seeded")
+            .unwrap_or(false)
+        {
+            return Ok(0);
         }
 
         let mut inserted = 0_usize;

src-tauri/src/database/dao/settings.rs

@@ -33,6 +33,18 @@ impl Database {
         }
     }
 
+    /// 以布尔语义读取 flag：`"true"` 或 `"1"` → true，其它全部 false。
+    ///
+    /// 用于一次性启动 flag（`official_providers_seeded` / `first_run_notice_shown` 等）。
+    /// 与 `is_legacy_common_config_migrated` 等只认 `"true"` 的历史辅助函数**不同**——
+    /// 这里同时接受 `"1"` 是为了兼容 `init_default_official_providers` 既有写法。
+    pub fn get_bool_flag(&self, key: &str) -> Result<bool, AppError> {
+        Ok(matches!(
+            self.get_setting(key)?.as_deref(),
+            Some("true") | Some("1")
+        ))
+    }
+
     /// 设置值
     pub fn set_setting(&self, key: &str, value: &str) -> Result<(), AppError> {
         let conn = lock_conn!(self.conn);