docs(deploy): update README and .env.example for Docker deployment

- Refresh README feature list and remove references to the legacy
  in-app monitoring charts / backend poller.
- Document configurable dashboard widgets, addon pages, and widget env vars.
- Add VITE_PROMETHEUS_URL support to frontend Dockerfile and both compose files.
- Add header comment to .env.example explaining shell-export workflow.
- Update remote server requirements to match current capabilities.

frontend/Dockerfile

@@ -16,6 +16,7 @@ ARG VITE_OIDC_REDIRECT_URI=
 ARG VITE_OIDC_POST_LOGOUT_REDIRECT_URI=
 ARG VITE_DEV_API_PROXY_TARGET=http://backend:8000
 ARG VITE_GRAFANA_URL=https://grafana.example.com
+ARG VITE_PROMETHEUS_URL=http://localhost:9090
 ARG VITE_APP_VERSION=0.1.0
 ARG VITE_APP_BUILD_INFO=dev
 
@@ -28,6 +29,7 @@ ENV VITE_API_URL=${VITE_API_URL} \
     VITE_OIDC_POST_LOGOUT_REDIRECT_URI=${VITE_OIDC_POST_LOGOUT_REDIRECT_URI} \
     VITE_DEV_API_PROXY_TARGET=${VITE_DEV_API_PROXY_TARGET} \
     VITE_GRAFANA_URL=${VITE_GRAFANA_URL} \
+    VITE_PROMETHEUS_URL=${VITE_PROMETHEUS_URL} \
     VITE_APP_VERSION=${VITE_APP_VERSION} \
     VITE_APP_BUILD_INFO=${VITE_APP_BUILD_INFO}
 
@@ -53,6 +55,7 @@ ENV VITE_API_URL=/api \
     VITE_OIDC_ENABLED=false \
     VITE_DEV_API_PROXY_TARGET=http://backend:8000 \
     VITE_GRAFANA_URL=http://localhost:3000 \
+    VITE_PROMETHEUS_URL=http://localhost:9090 \
     VITE_APP_VERSION=0.1.0 \
     VITE_APP_BUILD_INFO=dev