Add one-command setup via nix run

- Add flake app so setup can be run with: nix run .
- Update README with comprehensive setup guide for:
  - First computer (initial setup)
  - Additional computers (joining)
  - Mobile device pairing
  - NixOS module usage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -11,15 +11,78 @@ A NixOS-based system for managing the three types of data in a computer:
 ## Quick Start
 
 ```bash
-# Clone this repo
+# One-command setup (interactive)
+nix run github:YOUR_USERNAME/ultimate-notetaking-sync-backup-system
+
+# Or clone first
+git clone https://github.com/YOUR_USERNAME/ultimate-notetaking-sync-backup-system.git
+cd ultimate-notetaking-sync-backup-system
+nix run .
+```
+
+## Full Setup Guide
+
+### First Computer (Initial Setup)
+
+```bash
+# 1. Clone the repo
 git clone https://github.com/YOUR_USERNAME/ultimate-notetaking-sync-backup-system.git
 cd ultimate-notetaking-sync-backup-system
 
-# Option 1: Just try the tools (no system changes)
-nix develop
+# 2. Run setup (one command - includes all dependencies)
+nix run .
+# Or: nix develop && ./setup
 
-# Option 2: Apply as a NixOS module
-# Add to your flake.nix inputs, then import the module
+# 3. Choose option [1] "New setup (first device)"
+#    This generates an age encryption key - SAVE IT!
+
+# 4. Push config to your git server
+cd ~/.config/usync/config
+git remote add origin git@your-server:you/config.git
+git push -u origin main
+```
+
+### Additional Computers (Joining)
+
+```bash
+# One command with your git URL and age key
+nix run github:YOUR_USERNAME/ultimate-notetaking-sync-backup-system
+# Choose option [2], enter your git URL and age key
+
+# Or non-interactively:
+nix develop
+./setup <git-url> <your-age-key>
+```
+
+### Mobile Device (Phone/Tablet)
+
+```bash
+# On any computer that's already set up:
+nix run . -- mobile
+
+# Or: ./setup mobile
+```
+
+This shows a QR code for Syncthing pairing. You can also manually paste device IDs.
+
+### Just Try the Tools (No Setup)
+
+```bash
+nix develop
+# Provides: nb, syncthing, restic, rclone, age, sops, etc.
+```
+
+### Apply as NixOS Module
+
+Add to your flake.nix inputs, then import the module:
+
+```nix
+{
+  inputs.unsbs.url = "github:YOUR_USERNAME/ultimate-notetaking-sync-backup-system";
+
+  # In your configuration:
+  imports = [ inputs.unsbs.nixosModules.default ];
+}
 ```
 
 ## Philosophy