feat(failover): add auto-failover master switch with proxy integration (#427)

* feat(failover): add auto-failover master switch with proxy integration

- Add persistent auto_failover_enabled setting in database
- Add get/set_auto_failover_enabled commands
- Provider router respects master switch state
- Proxy shutdown automatically disables failover
- Enabling failover auto-starts proxy server
- Optimistic updates for failover queue toggle

* feat(proxy): persist proxy takeover state across app restarts

- Add proxy_takeover_{app_type} settings for per-app state tracking
- Restore proxy takeover state automatically on app startup
- Preserve state on normal exit, clear on manual stop
- Add stop_with_restore_keep_state method for graceful shutdown

* fix(proxy): set takeover state for all apps in start_with_takeover

src-tauri/src/commands/failover.rs

@@ -82,3 +82,28 @@ pub async fn set_failover_item_enabled(
         .set_failover_item_enabled(&app_type, &provider_id, enabled)
         .map_err(|e| e.to_string())
 }
+
+/// 获取自动故障转移总开关状态
+#[tauri::command]
+pub async fn get_auto_failover_enabled(state: tauri::State<'_, AppState>) -> Result<bool, String> {
+    state
+        .db
+        .get_setting("auto_failover_enabled")
+        .map(|v| v.map(|s| s == "true").unwrap_or(false)) // 默认关闭
+        .map_err(|e| e.to_string())
+}
+
+/// 设置自动故障转移总开关状态
+#[tauri::command]
+pub async fn set_auto_failover_enabled(
+    state: tauri::State<'_, AppState>,
+    enabled: bool,
+) -> Result<(), String> {
+    state
+        .db
+        .set_setting(
+            "auto_failover_enabled",
+            if enabled { "true" } else { "false" },
+        )
+        .map_err(|e| e.to_string())
+}