docs(desktop): add README with setup, config, and build steps

surfsense_desktop/README.md

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+# SurfSense Desktop
+
+Electron wrapper around the SurfSense web app. Packages the Next.js standalone build into a native desktop application with OAuth support, deep linking, and system browser integration.
+
+## Prerequisites
+
+- Node.js 18+
+- pnpm 10+
+- The `surfsense_web` project dependencies installed (`pnpm install` in `surfsense_web/`)
+
+## Development
+
+```bash
+pnpm install
+pnpm dev
+```
+
+This starts the Next.js dev server and Electron concurrently. Hot reload works — edit the web app and changes appear immediately.
+
+## Configuration
+
+Two `.env` files control the build:
+
+**`surfsense_web/.env`** — Next.js environment variables baked into the frontend at build time:
+
+```
+NEXT_PUBLIC_FASTAPI_BACKEND_URL=https://your-deployed-backend.com
+NEXT_PUBLIC_ELECTRIC_URL=https://your-deployed-electric.com
+NEXT_PUBLIC_ELECTRIC_AUTH_MODE=secure
+```
+
+**`surfsense_desktop/.env`** — Electron-specific configuration:
+
+```
+HOSTED_FRONTEND_URL=https://surfsense.net
+```
+
+Set these before building.
+
+## Build & Package
+
+**Step 1** — Build the Next.js standalone output:
+
+```bash
+cd ../surfsense_web
+pnpm next build
+```
+
+**Step 2** — Compile Electron and prepare the standalone output:
+
+```bash
+cd ../surfsense_desktop
+pnpm build
+```
+
+**Step 3** — Package into a distributable:
+
+```bash
+pnpm dist:mac      # macOS (.dmg + .zip)
+pnpm dist:win      # Windows (.exe)
+pnpm dist:linux    # Linux (.deb + .AppImage)
+```
+
+**Step 4** — Find the output:
+
+```bash
+ls release/
+```