[codex] Resize gridster when its host element changes

[nexiumbiz-debug]

Summary

• observe the gridster host element with ResizeObserver when available
• call the existing resize path when the element changes size, so collapsible containers/layout panels are handled without relying on window resize
• disconnect the observer during component teardown
• add regression coverage for observer creation, resize callback forwarding, and cleanup

Fixes #824

Validation

• npx prettier --check projects/angular-gridster2/src/lib/gridster.ts projects/angular-gridster2/src/lib/tests/gridster.spec.ts
• npx eslint projects/angular-gridster2/src/lib/tests/gridster.spec.ts
• git diff --check
• npm run test-lib -- --watch=false
• npm run build-lib