fix: prevent common config loss during proxy takeover and stabilize snippet lifecycle

- Make sync_current_provider_for_app takeover-aware: update restore
  backup instead of overwriting live config when proxy is active
- Introduce explicit "cleared" flag for common config snippets to
  prevent auto-extraction from resurrecting user-cleared snippets
- Reorder startup: extract snippets from clean live files before
  restoring proxy takeover state
- Add one-time migration flag to skip legacy commonConfigEnabled
  migration on subsequent startups
- Add regression tests for takeover backup preservation, explicit
  clear semantics, and migration flag roundtrip

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

@@ -7,6 +7,12 @@ use crate::error::AppError;
 use rusqlite::params;
 
 impl Database {
+    const LEGACY_COMMON_CONFIG_MIGRATED_KEY: &'static str = "common_config_legacy_migrated_v1";
+
+    fn config_snippet_cleared_key(app_type: &str) -> String {
+        format!("common_config_{app_type}_cleared")
+    }
+
     /// 获取设置值
     pub fn get_setting(&self, key: &str) -> Result<Option<String>, AppError> {
         let conn = lock_conn!(self.conn);
@@ -45,6 +51,60 @@ impl Database {
         self.get_setting(&format!("common_config_{app_type}"))
     }
 
+    /// 检查通用配置片段是否被用户显式清空
+    pub fn is_config_snippet_cleared(&self, app_type: &str) -> Result<bool, AppError> {
+        Ok(self
+            .get_setting(&Self::config_snippet_cleared_key(app_type))?
+            .as_deref()
+            == Some("true"))
+    }
+
+    /// 设置通用配置片段是否被显式清空
+    pub fn set_config_snippet_cleared(
+        &self,
+        app_type: &str,
+        cleared: bool,
+    ) -> Result<(), AppError> {
+        let key = Self::config_snippet_cleared_key(app_type);
+        if cleared {
+            self.set_setting(&key, "true")
+        } else {
+            let conn = lock_conn!(self.conn);
+            conn.execute("DELETE FROM settings WHERE key = ?1", params![key])
+                .map_err(|e| AppError::Database(e.to_string()))?;
+            Ok(())
+        }
+    }
+
+    /// 当前是否允许从 live 配置自动抽取通用配置片段
+    pub fn should_auto_extract_config_snippet(&self, app_type: &str) -> Result<bool, AppError> {
+        Ok(self.get_config_snippet(app_type)?.is_none()
+            && !self.is_config_snippet_cleared(app_type)?)
+    }
+
+    /// 检查历史通用配置迁移是否已经执行过
+    pub fn is_legacy_common_config_migrated(&self) -> Result<bool, AppError> {
+        Ok(self
+            .get_setting(Self::LEGACY_COMMON_CONFIG_MIGRATED_KEY)?
+            .as_deref()
+            == Some("true"))
+    }
+
+    /// 标记历史通用配置迁移已经执行完成
+    pub fn set_legacy_common_config_migrated(&self, migrated: bool) -> Result<(), AppError> {
+        if migrated {
+            self.set_setting(Self::LEGACY_COMMON_CONFIG_MIGRATED_KEY, "true")
+        } else {
+            let conn = lock_conn!(self.conn);
+            conn.execute(
+                "DELETE FROM settings WHERE key = ?1",
+                params![Self::LEGACY_COMMON_CONFIG_MIGRATED_KEY],
+            )
+            .map_err(|e| AppError::Database(e.to_string()))?;
+            Ok(())
+        }
+    }
+
     /// 设置通用配置片段
     pub fn set_config_snippet(
         &self,