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/services/provider/mod.rs

@@ -614,6 +614,46 @@ impl ProviderService {
         state: &AppState,
         app_type: AppType,
     ) -> Result<(), AppError> {
+        if app_type.is_additive_mode() {
+            return sync_current_provider_for_app_to_live(state, &app_type);
+        }
+
+        let current_id =
+            match crate::settings::get_effective_current_provider(&state.db, &app_type)? {
+                Some(id) => id,
+                None => return Ok(()),
+            };
+
+        let providers = state.db.get_all_providers(app_type.as_str())?;
+        let Some(provider) = providers.get(&current_id) else {
+            return Ok(());
+        };
+
+        let takeover_enabled =
+            futures::executor::block_on(state.db.get_proxy_config_for_app(app_type.as_str()))
+                .map(|config| config.enabled)
+                .unwrap_or(false);
+
+        let has_live_backup =
+            futures::executor::block_on(state.db.get_live_backup(app_type.as_str()))
+                .ok()
+                .flatten()
+                .is_some();
+
+        let live_taken_over = state
+            .proxy_service
+            .detect_takeover_in_live_config_for_app(&app_type);
+
+        if takeover_enabled && (has_live_backup || live_taken_over) {
+            futures::executor::block_on(
+                state
+                    .proxy_service
+                    .update_live_backup_from_provider(app_type.as_str(), provider),
+            )
+            .map_err(|e| AppError::Message(format!("更新 Live 备份失败: {e}")))?;
+            return Ok(());
+        }
+
         sync_current_provider_for_app_to_live(state, &app_type)
     }