L8 grouping: zynthian-ui sync_chain sends 'plan' and shows the reason of a plugin left alone #190

Lane L8 (repo FreeMixer/zynthian-ui, which has issues disabled, so it is tracked here). Needs L2 (#185). Branch base feat/ph-default-and-qualified-menus. Fall back to chain against a daemon without the plan verb; remove CHAIN_FALLBACK_CODES and chain_refused once the daemon's reason replaces them.

Design: https://github.com/FreeMixer/plugin-hostd/blob/main/docs/design/notes/2026-10-27-grouping-planner.md (PR #183). Part of #182, FreeMixer/plugin-qualify#157, FreeMixer/openmixer#2263.