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/i18n/locales/zh.json

@@ -123,6 +123,9 @@
     "importing": "导入中...",
     "importSuccess": "导入成功！",
     "importFailed": "导入失败",
+    "syncLiveFailed": "已导入，但同步到当前供应商失败，请手动重新选择一次供应商。",
+    "importPartialSuccess": "配置已导入，但同步到当前供应商失败。",
+    "importPartialHint": "请手动重新选择一次供应商以刷新对应配置。",
     "configExported": "配置已导出到：",
     "exportFailed": "导出失败",
     "selectFileFailed": "选择文件失败",