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/src/preload.js

@@ -7,9 +7,12 @@ contextBridge.exposeInMainWorld("electronAPI", {
   // Navigation
   navigate: (page) => ipcRenderer.invoke("navigate", page),
 
-  // Sidecar status
+  // Sidecar status (Whisper transcriber)
   getSidecarStatus: () => ipcRenderer.invoke("get-sidecar-status"),
 
+  // Backend status (FastAPI server)
+  getBackendStatus: () => ipcRenderer.invoke("get-backend-status"),
+
   // === Streaming Mode APIs ===
   startRecordingStream: () => ipcRenderer.invoke("start-recording-stream"),
   streamAudioChunk: (base64Audio) => ipcRenderer.invoke("stream-audio-chunk", base64Audio),
@@ -26,6 +29,11 @@ contextBridge.exposeInMainWorld("electronAPI", {
     ipcRenderer.on("stream-stopped", (event, data) => callback(data));
   },
 
+  // Model download progress events
+  onModelDownloadProgress: (callback) => {
+    ipcRenderer.on("model-download-progress", (event, progress) => callback(progress));
+  },
+
   // === Legacy File-based APIs (fallback) ===
   saveAudioFile: (arrayBuffer) => ipcRenderer.invoke("save-audio-file", arrayBuffer),
   transcribeAudio: (filePath) => ipcRenderer.invoke("transcribe-audio", filePath),