fix: prevent silent config fallback and data loss on startup

This commit introduces fail-fast error handling for config loading failures,
replacing the previous silent fallback to default config which could cause
data loss (e.g., all user providers disappearing).

Key changes:

Backend (Rust):
- Replace AppState::new() with AppState::try_new() to explicitly propagate errors
- Remove Default trait to prevent accidental empty state creation
- Add init_status module as global error cache (OnceLock + RwLock)
- Implement dual-channel error notification:
  1. Event emission (low-latency, may race with frontend subscription)
  2. Command-based polling (reliable, guaranteed delivery)
- Remove unconditional save on startup to prevent overwriting corrupted config

Frontend (TypeScript):
- Add event listener for "configLoadError" (fast path)
- Add bootstrap-time polling via get_init_error command (reliable path)
- Display detailed error dialog with recovery instructions
- Prompt user to exit for manual repair

Impact:
- First-time users: No change (load() returns Ok(default) when file missing)
- Corrupted config: Application shows error and exits gracefully
- Prevents accidental config overwrite during initialization

Fixes the only critical issue identified in previous code review (silent
fallback causing data loss).

src-tauri/src/store.rs

@@ -7,23 +7,14 @@ pub struct AppState {
     pub config: RwLock<MultiAppConfig>,
 }
 
-impl Default for AppState {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
 impl AppState {
     /// 创建新的应用状态
-    pub fn new() -> Self {
-        let config = MultiAppConfig::load().unwrap_or_else(|e| {
-            log::warn!("加载配置失败: {}, 使用默认配置", e);
-            MultiAppConfig::default()
-        });
-
-        Self {
+    /// 注意：仅在配置成功加载时返回；不会在失败时回退默认值。
+    pub fn try_new() -> Result<Self, AppError> {
+        let config = MultiAppConfig::load()?;
+        Ok(Self {
             config: RwLock::new(config),
-        }
+        })
     }
 
     /// 保存配置到文件