feat: sync app settings across devices via Matrix account data

[Just-Insane]

Description

Adds cross-device settings sync by storing Sable's app settings in a Matrix account data event (moe.sable.app.settings). Change a setting on one device and it propagates to your other sessions automatically.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

What the generated code does:

settingsSync.ts — Serialise/deserialise helpers that convert the settings state to/from the account data wire format. Unknown keys are preserved so older clients don't silently drop data from newer ones.

useSettingsSync.ts — The main sync hook. On mount it reads any existing account data and merges it into local state. It then listens for live updates from other devices via ClientEvent.AccountData. Uploads are debounced by 2 seconds and each one embeds a random synctoken; when the server echoes the event back, the hook recognises its own token and skips re-applying it. Exposes settingsSyncStatusAtom and settingsSyncLastSyncedAtom for UI feedback.

General.tsx — Adds a "Sync settings across devices" toggle and a JSON export/import section for manually backing up and restoring settings.

ClientNonUIFeatures.tsx — Mounts the sync hook once at the client root so it stays active for the whole session.

settings.ts — Adds settingsSyncEnabled: boolean (default false) to the settings atom.

Type files — Registers moe.sable.app.settings in the account data type registry so SDK callbacks are typed correctly.

Tests cover the serialise/deserialise helpers, load-on-mount, merge behaviour, echo-loop skipping, debounced uploads, and error states.

[dozro]

Syncs Sable application settings across devices using Matrix account data (custom event type app.sable.settings)

shouldn’t the event type be moe.sable.xyz and not app.sable.xyz?

as we don’t have the right to the app.sable namespace, see https://spec.matrix.org/unstable/#namespacing

[Just-Insane]

Syncs Sable application settings across devices using Matrix account data (custom event type app.sable.settings)

shouldn’t the event type be moe.sable.xyz and not app.sable.xyz?

Hmm, probably yea - actually, it's correct in the source, just wrong in the description

[Just-Insane]

@dozro should be good now for those changes.

[dozro]

code lgtm now :3