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/package.json

@@ -33,6 +33,11 @@
         "to": "sidecar/transcriber",
         "filter": ["**/*"]
       },
+      {
+        "from": "../backend/dist/backend",
+        "to": "backend/backend",
+        "filter": ["**/*"]
+      },
       {
         "from": "config.json",
         "to": "config.json"