feat: 添加 Tauri Mac 原生应用支持

- 新增 mac-app/ 目录：Tauri 项目结构（Rust 外壳 + sidecar 进程管理）
- 前端添加 Tauri 环境检测，API 调用和 WebSocket 自动适配
- 后端支持 MEETING_DATA_DIR 环境变量指定数据目录
- 新增 /api/meetings/{id}/retranscribe 接口用于重新转录
- 修复轮询超时：2分钟→30分钟，适配长音频
- 添加 .gitignore 排除构建产物

mac-app/src-tauri/tauri.conf.json

@@ -0,0 +1,44 @@
+{
+  "$schema": "https://raw.githubusercontent.com/tauri-apps/tauri/dev/crates/tauri-config-schema/schema.json",
+  "productName": "Meeting Recorder",
+  "version": "1.0.0",
+  "identifier": "com.meeting.recorder",
+  "build": {
+    "frontendDist": "../../frontend/dist",
+    "devUrl": "http://localhost:3000",
+    "beforeDevCommand": "cd ../frontend && npm run dev",
+    "beforeBuildCommand": "cd ../frontend && npm run build"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "智能会议记录系统",
+        "width": 1200,
+        "height": 800,
+        "minWidth": 900,
+        "minHeight": 600,
+        "resizable": true,
+        "fullscreen": false
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": ["dmg", "app"],
+    "externalBin": ["binaries/meeting-backend"],
+    "resources": [],
+    "macOS": {
+      "minimumSystemVersion": "12.0",
+      "infoPlist": "Info.plist"
+    },
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns"
+    ]
+  }
+}