feat: Add embedded backend packaging for all-in-one deployment

- Add backend/run_server.py entry point for embedded deployment
- Add backend/build.py PyInstaller script for backend packaging
- Modify config.py to support frozen executable paths
- Extend client/config.json with backend configuration section
- Add backend sidecar management in Electron main process
- Add Whisper model download progress reporting
- Update build-client.bat with --embedded-backend flag
- Update DEPLOYMENT.md with all-in-one deployment documentation

This enables packaging frontend and backend into a single executable
for simplified enterprise deployment. Backward compatible with
existing separate deployment mode (backend.embedded: false).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/config.json

@@ -6,5 +6,28 @@
     "model": "medium",
     "device": "cpu",
     "compute": "int8"
+  },
+  "backend": {
+    "embedded": false,
+    "host": "127.0.0.1",
+    "port": 8000,
+    "database": {
+      "host": "mysql.theaken.com",
+      "port": 33306,
+      "user": "",
+      "password": "",
+      "database": ""
+    },
+    "externalApis": {
+      "authApiUrl": "https://pj-auth-api.vercel.app/api/auth/login",
+      "difyApiUrl": "https://dify.theaken.com/v1",
+      "difyApiKey": "",
+      "difySttApiKey": ""
+    },
+    "auth": {
+      "adminEmail": "",
+      "jwtSecret": "",
+      "jwtExpireHours": 24
+    }
   }
 }