docs: rewrite README with full structure, Docker, setup guide, v1.2.0

- Add complete project structure section
- Add npm commands (dev/build/preview) and port info
- Add Docker deployment section
- Add new customer setup guide with customization steps
- Fix package.json version from 1.0.0 to 1.2.0
- Add version info referencing VERSIONS.json

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "frontend",
   "private": true,
-  "version": "1.0.0",
+  "version": "1.2.0",
   "type": "module",
   "scripts": {
     "dev": "vite",