feat: sync current providers to live files after config import

Core Improvements:
- Add sync_current_providers_live command to synchronize in-memory provider
  settings to corresponding live files (~/.claude/settings.json or ~/.codex/auth.json)
- Introduce partial-success state to distinguish between 'import succeeded
  but sync failed' scenario, providing clear user feedback
- Remove unused skip_live_backfill parameter from switch_provider command
- Separate responsibilities: backend handles import/backup, frontend handles
  sync/error presentation

Technical Details:
- Codex: sync auth.json + config.toml with MCP configuration
- Claude: sync settings.json
- Bidirectional sync: read back after write to update in-memory settings_config
- Full i18n support (English and Chinese)
- Graceful handling when no current provider is active

Affected Files:
- Backend: import_export.rs, commands.rs, lib.rs
- Frontend: useImportExport.ts, ImportExportSection.tsx, settings.ts
- i18n: en.json, zh.json

This ensures SSOT (Single Source of Truth) consistency between config.json
and live configuration files after import operations.

src/lib/api/settings.ts

@@ -97,6 +97,16 @@ export const settingsApi = {
     });
   },
 
+  async syncCurrentProvidersLive(): Promise<void> {
+    const result = (await invoke("sync_current_providers_live")) as {
+      success?: boolean;
+      message?: string;
+    };
+    if (!result?.success) {
+      throw new Error(result?.message || "Sync current providers failed");
+    }
+  },
+
   async openExternal(url: string): Promise<void> {
     try {
       const u = new URL(url);