fix: resolve React lint errors in canvas and save-template-overlay

- Replace useState/useEffect mounted pattern with useSyncExternalStore
  for hydration-safe client detection
- Wrap setAgentState in useCallback to stabilize effect dependencies
- Replace ref-during-render with state+effect for pending_template capture
- Fix ConfirmChangesProps types (remove any, drop unused args prop)
- Add eslint-disable for intentionally scoped effect dependencies

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

Author: GeneralJerel