docs: update electron/README.md, USER_GUIDE.md, FEATURES.md with desktop app docs

docs/USER_GUIDE.md

@@ -755,3 +755,55 @@ Access via **Dashboard → Health**. Real-time system health overview with 6 car
 | **Latency Telemetry** | p50/p95/p99 latency aggregation per provider                |
 
 **Pro Tip:** The Health page auto-refreshes every 10 seconds. Use the circuit breaker card to identify which providers are experiencing issues.
+
+---
+
+## 🖥️ Desktop Application (Electron)
+
+OmniRoute is available as a native desktop application for Windows, macOS, and Linux.
+
+### Installation
+
+```bash
+# From the electron directory:
+cd electron
+npm install
+
+# Development mode (connect to running Next.js dev server):
+npm run dev
+
+# Production mode (uses standalone build):
+npm start
+```
+
+### Building Installers
+
+```bash
+cd electron
+npm run build          # Current platform
+npm run build:win      # Windows (.exe NSIS)
+npm run build:mac      # macOS (.dmg universal)
+npm run build:linux    # Linux (.AppImage)
+```
+
+Output → `electron/dist-electron/`
+
+### Key Features
+
+| Feature                     | Description                                          |
+| --------------------------- | ---------------------------------------------------- |
+| **Server Readiness**        | Polls server before showing window (no blank screen) |
+| **System Tray**             | Minimize to tray, change port, quit from tray menu   |
+| **Port Management**         | Change server port from tray (auto-restarts server)  |
+| **Content Security Policy** | Restrictive CSP via session headers                  |
+| **Single Instance**         | Only one app instance can run at a time              |
+| **Offline Mode**            | Bundled Next.js server works without internet        |
+
+### Environment Variables
+
+| Variable              | Default | Description                      |
+| --------------------- | ------- | -------------------------------- |
+| `OMNIROUTE_PORT`      | `20128` | Server port                      |
+| `OMNIROUTE_MEMORY_MB` | `512`   | Node.js heap limit (64–16384 MB) |
+
+📖 Full documentation: [`electron/README.md`](../electron/README.md)