Mobile: creating/renaming/deleting a channel section (or moving a channel into one) doesn't update the UI

Start in mobile/lib/features/channels/channel_sections/channel_sections_manager.dart at the listed mutation methods, then read channel_sections_provider.dart and the ref.watch call in mobile/lib/features/channels/channels_page/body.dart. Reproduce the create-section flow on mobile and verify that creating, renaming, deleting, reordering, and channel assignment changes are immediately reflected in the sidebar.

Summary
On mobile, local mutations to channel sections (create, rename, delete, reorder, assign/unassign a channel) are persisted to disk and eventually published to the relay, but the sidebar UI does not reflect them. The section list appears unchanged — most visibly, creating a new section via "Move to section... -> New section..." shows no result at all.

Environment

• Client: Buzz mobile (iOS, Flutter)
• Repo commit: 45f4b91a3

Steps to reproduce

1. Open a channel, tap "Move to section..."
2. Tap "New section...", enter a name, tap "Create"
3. Observe: nothing happens — no new section appears in the sidebar

Same symptom for renaming, deleting, reordering a section, or assigning/removing a channel from one.

Root cause
mobile/lib/features/channels/channel_sections/channel_sections_manager.dart — ChannelSectionsManager mutation methods (createSection L191, renameSection L210, deleteSection L231, moveSectionUp L246, moveSectionDown L255, assignChannel L264, unassignChannel L276) update _store and call _persist() + markDirty() (which only schedules a debounced relay publish), but never call _onChanged().

_onChanged() is what drives ChannelSectionsNotifier._emitManagerState() in channel_sections_provider.dart, which reassigns Riverpod state. The sidebar widget (mobile/lib/features/channels/channels_page/body.dart:104, ref.watch(channelSectionsProvider)) only rebuilds when that state object changes — so local mutations are silently invisible until some unrelated event (relay fetch, live subscription update) happens to call _onChanged() later.

Suggested fix
Call _onChanged() in each of the mutation methods above, right after _persist().