feat: OpenMesh 基础平台与 MD/PDF 转换技能

- 后端: coworker 智能体框架, WS API, 文件上传, 附件处理
- 前端: Open WebUI, 文件全量走 upload API (含 MD/TXT/JSON 等文本类)
- 技能: md-to-office (pandoc + wkhtmltopdf)
- 修复: 上传文件路径丢失, Agent 搜索浪费, 输出文件跑到 uploads/
- 打包: PyInstaller one-dir, 预打包 pandoc/wkhtmltopdf/chromium

surfaces/gui/src-tauri/build.rs

@@ -0,0 +1,10 @@
+fn main() {
+    // tauri-build validates every `bundle.resources` path on every build, dev included, but
+    // `binaries/sidecar` is only staged by the release scripts and `/binaries` is gitignored —
+    // so a fresh checkout died on `resource path 'binaries/sidecar' doesn't exist`. Dev needs no
+    // packaged server (`server_bin()` falls back to the venv) and empty resource dirs are
+    // skipped, so a placeholder is enough.
+    std::fs::create_dir_all("binaries/sidecar").expect("create the sidecar resource dir");
+
+    tauri_build::build()
+}