fix(export): expand group system messages and add Rust exporter

[jingkarqi]

Summary

Fixes #877

This PR focuses on export correctness and performance:

• Fixes group chat system-message export where WeChat template placeholders such as $username$ and $names$ were exported literally instead of being expanded to readable member names.
• Adds a Rust-based streaming exporter for large text-only exports, with an engine preference setting (Auto / Rust / TypeScript).
• Adds a TypeScript streaming export path for supported text formats and keeps the existing TypeScript exporter as the compatibility path for media/avatar/voice-transcription/full-format exports.
• Shows the active export engine in task progress so users can tell whether a task is using Rust, TypeScript, or a Rust-to-TypeScript fallback.
• Adds Vitest coverage for the new formatter, engine routing, streaming message reads, writers, pause/cancel handling, and large synthetic exports.

What Changed

Fix group system-message placeholders

• Extracted shared system-message formatting into systemMessageFormatter.
• Reused the formatter from both chat display/export paths.
• Added support for resolving system-message templates that carry member data in <link name="..."> / <member> structures.
• Covers invitation and QR-code join messages so exported content becomes readable, for example:
  • before: "$username$"邀请"$names$"加入了群聊
  • after: "张三"邀请"李四、王五"加入了群聊

Add Rust export engine

• Added a native Rust exporter project under native/weflow-exporter.
• Added scripts/build-rust-exporter.cjs to build and copy the native binary into resources/exporter/<platform>/<arch>.
• Added the Windows x64 exporter binary resource.
• Rust is used for text-only supported formats: JSON, TXT, HTML, ChatLab JSONL, and WeClone.

Export engine routing and fallback

• Added a central export-engine router.
• Auto mode chooses Rust only when the selected export is supported and text-only.
• Media, avatar, voice, file, emoji, and voice-to-text exports remain on TypeScript for compatibility.
• In Auto, Rust failure falls back to TypeScript; manually selecting Rust keeps failure strict.
• Pause/cancel signals are preserved and are not treated as Rust failures.

UI and config

• Added export engine preference to export defaults.
• Added engine labels to export task progress.
• Avoided silently rewriting user export options in the renderer; backend routing now decides the engine from the real user-selected options.

Verification

• npm run typecheck
• npm run test
• npm run build:rust-exporter
• npm run build produced release/win-unpacked/WeFlow.exe; local installer packaging was blocked by Windows symlink permission while electron-builder extracted winCodeSign, unrelated to app compilation.

[xunchahaha]

rust导出仅支持纯文字导出 42000条的会话我实测仅跟现有方案差了半秒左右 ·并且没有考虑多平台适配 chatlab的jsonl导出也破坏了格式 我觉得应该可以删了 仅保留群聊邀请消息的修复