fix(mobile): refresh channel sections after local mutations

[thesid42]

Summary

• notify mobile channel-section observers synchronously after local create, rename, delete, reorder, assign, and unassign mutations
• schedule debounced relay publication before observer callbacks so callback-triggered disposal still flushes updated state
• add regression coverage for persistence ordering, exactly-once notifications, move-boundary no-ops, and disposal-time publication

Related issue

Fixes #5797.

Testing

• cd mobile && flutter test test/features/channels/channel_sections/channel_sections_manager_test.dart test/features/channels/channel_actions_sheet_test.dart test/features/channels/channels_page_test.dart (57 passed)
• cd mobile && flutter test (1,467 passed)
• cd mobile && dart format --output=none --set-exit-if-changed . && flutter analyze
• node mobile/scripts/check-file-sizes.mjs
• pnpm run check

Screenshots: Not included. This fixes state propagation and does not change visual styling.

[Copilot]

Pull request overview

This PR fixes a mobile UI staleness issue where local channel-section mutations (create/rename/delete/reorder/assign/unassign) were persisted and scheduled for relay publication, but did not synchronously notify Riverpod observers—so the sidebar didn’t rebuild until a later, unrelated update.

Changes:

• Add synchronous _onChanged() notifications after local mutations, ensuring state observers update immediately.
• Ensure debounced relay publication is scheduled before observer callbacks so onChanged-triggered disposal can still flush pending publication.
• Add regression tests covering persistence-before-notify ordering, exactly-once sync notifications, no-op move boundaries, and disposal-time publish flush.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
mobile/lib/features/channels/channel_sections/channel_sections_manager.dart	Calls _onChanged() after local mutations (after persistence + debounced publish scheduling) so UI observers update immediately.
mobile/test/features/channels/channel_sections/channel_sections_manager_test.dart	Adds regression coverage for sync notifications, persistence ordering, move no-ops, and disposal-triggered publish flush.

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