feat: add version display to frontend and /api/version endpoint (v1.2.5)

- Add GET /api/version endpoint that reads VERSION file
- Frontend fetches version on init and displays it as a badge in the header
- Add version-badge CSS styling
- Update docker-compose.yml comment to v1.2.5

docker-compose.yml

@@ -14,7 +14,7 @@ services:
   backend:
     build: ./backend
     # For production, use the pre-built image instead:
-    # image: git.cqre.net/cqrenet/aoc-backend:v1.1.0
+    # image: git.cqre.net/cqrenet/aoc-backend:v1.2.5
     container_name: aoc-backend
     restart: always
     env_file: