feat: add semver auto-increment and version display

- Add VERSION file (starts at 0.1.0)
- Deploy workflow bumps patch on every push, commits back [skip ci]
- APP_VERSION passed into container via env
- Version shown in page header next to title

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -39,6 +39,7 @@ services:
     environment:
       - OUTLINE_URL=${OUTLINE_URL:-http://172.29.0.13:3000}
       - OUTLINE_TOKEN=${OUTLINE_TOKEN}
+      - APP_VERSION=${APP_VERSION:-dev}
 
 volumes:
   tailscale-state: