fix: 更新渠道默认值跟随安装包版本

从 GitHub 下载 Beta 安装包安装后，更新通道默认选中 Release，
因为 update-channel.json 状态文件不存在时会硬编码回退到 release。

新增 theseus::default_update_channel()，按编译期版本号判断：含预发布
后缀（如 1.0.2-beta7）默认 Beta，否则 Release。数据库渠道与设置页
渠道两处默认值统一改为调用它，保证两者一致。

packages/app-lib/src/lib.rs

@@ -33,7 +33,7 @@ pub use logger::start_logger;
 pub use state::db::{
     backup_current_app_db_for_update, beta_database_exists,
     copy_database_between_channels, copy_release_database_to_beta,
-    current_app_database_path,
+    current_app_database_path, default_update_channel,
 };
 pub use state::{DirectoryInfo, State};
 pub use storage::*;

packages/app-lib/src/state/db.rs

@@ -304,6 +304,18 @@ async fn app_db_path(settings_dir: &Path) -> crate::Result<PathBuf> {
     Ok(settings_dir.join(channel).join(LEGACY_APP_DB_FILE))
 }
 
+/// The update channel a fresh install should default to. Beta builds carry a
+/// pre-release version (e.g. `1.0.2-beta7`), so their installers should start
+/// on the Beta channel instead of silently falling back to Release.
+pub fn default_update_channel() -> &'static str {
+    let version = env!("CARGO_PKG_VERSION");
+    if version.contains('-') {
+        "beta"
+    } else {
+        "release"
+    }
+}
+
 async fn read_update_channel(
     settings_dir: &Path,
 ) -> crate::Result<&'static str> {
@@ -311,7 +323,7 @@ async fn read_update_channel(
     let contents = match tokio::fs::read_to_string(&path).await {
         Ok(contents) => contents,
         Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
-            return Ok("release");
+            return Ok(default_update_channel());
         }
         Err(error) => return Err(error.into()),
     };