Add web-app with machine-specific builds

Implements per-machine customization for web-app deployment:
- Shared web-app source code deployed to all machines
- Machine-specific .env files and images
- Build helper script to assemble and build on target machines

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

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

DEPLOYMENT-GUIDE.md

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+# Web-App Deployment Guide
+
+## Overview
+
+This setup allows each machine to build the web-app with its own specific configuration:
+- Machine-specific `.env` files
+- Machine-specific images in the `public` folder
+
+## Structure
+
+```
+.
+├── web-app/                          # Shared web-app source code
+│   ├── package.json
+│   ├── index.html
+│   └── public/                       # Images will be copied here during build
+├── machine-specific/
+│   ├── machine1/
+│   │   ├── env/.env                  # Machine1's environment file
+│   │   └── images/                   # Machine1's images
+│   │       ├── logo.png
+│   │       └── banner.jpg
+│   └── machine2/
+│       ├── env/.env                  # Machine2's environment file
+│       └── images/                   # Machine2's images
+│           ├── logo.png
+│           └── banner.jpg
+└── build-helper.nix                  # Build script available on target machines
+```
+
+## How It Works
+
+### 1. Krops Deployment (krops.nix)
+
+When you deploy with krops, it copies to each target machine:
+- `web-app` → `/var/src/web-app` (shared source code)
+- `machine-specific/{name}/env/.env` → `/var/src/web-app-env` (machine-specific .env)
+- `machine-specific/{name}/images/` → `/var/src/web-app-images/` (machine-specific images)
+
+### 2. Building on Target Machine
+
+After deployment, SSH into the target machine and run:
+
+```bash
+build-web-app
+```
+
+This script:
+1. Copies the machine-specific `.env` file to the web-app directory
+2. Copies the machine-specific images to `web-app/public/`
+3. Runs `npm run build` to build the web-app
+
+The build output will be in `/var/src/web-app/dist/`
+
+## Usage
+
+### Deploy to a single machine:
+```bash
+nix-build ./krops.nix -A machine1 && ./result
+```
+
+### Deploy to all machines:
+```bash
+nix-build ./krops.nix -A all && ./result
+```
+
+### Build the web-app on target machine:
+```bash
+ssh root@machine1.example.com
+build-web-app
+```
+
+## Customization
+
+### Add a new machine:
+1. Create directories: `machine-specific/machine3/env/` and `machine-specific/machine3/images/`
+2. Add `.env` file and images for machine3
+3. Create `config/machine3/configuration.nix`
+4. Add machine3 target in `krops.nix`
+
+### Update environment variables:
+Edit the appropriate `.env` file in `machine-specific/{machine-name}/env/.env`
+
+### Update images:
+Replace files in `machine-specific/{machine-name}/images/`
+
+After making changes, redeploy with krops.