Add one-command setup with mobile QR pairing

- Add ./setup script for single-command device initialization
  - Interactive menu: new setup, join existing, or pair mobile
  - Bootstrap from git repo + age key: ./setup <url> <key>
  - Mobile pairing with live camera QR scanning

- Enhance usync with device management commands
  - usync init: bootstrap from config repo
  - usync invite: show QR for mobile
  - usync add-device: add device by ID
  - usync join: join existing setup

- Mobile/GrapheneOS optimizations
  - Live camera preview with targeting box (ffmpeg + SDL)
  - Auto-detect QR codes without manual capture
  - Syncthing pending device detection

- Simplify nix develop shell message
- Add ffmpeg, zbar, qrencode, age, sops to devShell

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

flake.nix

@@ -94,6 +94,13 @@
               restic          # Backup
               rclone          # Cloud storage
 
+              # Setup & Pairing
+              qrencode        # QR codes for mobile pairing
+              zbar            # QR scanning (zbarimg)
+              ffmpeg          # Camera preview + capture
+              age             # Encryption for secrets
+              sops            # Secret management
+
               # Development
               git
               neovim
@@ -107,16 +114,12 @@
             ];
 
             shellHook = ''
-              echo "Ultimate Notetaking, Sync & Backup System"
+              printf '\033[1m%s\033[0m\n' "Ultimate Notetaking, Sync & Backup System"
               echo ""
-              echo "Available tools:"
-              echo "  nb        - Note-taking CLI"
-              echo "  syncthing - File synchronization"
-              echo "  unison    - Alternative file sync"
-              echo "  restic    - Backup"
-              echo "  rclone    - Cloud storage"
-              echo ""
-              echo "Run 'nb help' to get started with notes."
+              echo "Get started:  ./setup"
+              echo "Pair mobile:  ./setup mobile"
+              echo "Sync:         ./scripts/usync"
+              echo "Status:       ./scripts/ustatus"
             '';
           };
         }