docs: add app screenshots, default compose to published :latest images

- docker-compose.yml now references ghcr.io/dvorinka/containr-*:latest with
  pull_policy: missing, so `docker compose up` pulls published images by
  default; CONTAINR_VERSION pins a release tag, build config retained for
  source builds
- install.sh pulls instead of always building (CONTAINR_BUILD=1 or a failed
  pull falls back to a source build)
- README gains a Screenshots section (dashboard, canvas, service detail,
  databases) captured from a live local stack, plus updated install docs

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docker-compose.yml

@@ -33,6 +33,8 @@ services:
     restart: unless-stopped
 
   backend:
+    image: ghcr.io/dvorinka/containr-backend:${CONTAINR_VERSION:-latest}
+    pull_policy: missing
     build:
       context: ./app/backend
       dockerfile: Dockerfile
@@ -94,6 +96,8 @@ services:
       start_period: 30s
 
   frontend:
+    image: ghcr.io/dvorinka/containr-frontend:${CONTAINR_VERSION:-latest}
+    pull_policy: missing
     build:
       context: ./app/frontend
       dockerfile: Dockerfile