feat: sync settings across devices via Matrix account data

[Just-Insane]

Closes #402

What

Adds opt-in settings synchronisation across devices using Matrix account data (moe.sable.app.settings), plus a JSON export/import fallback for users who prefer manual control.

How it works

• Sync toggle (off by default) in General settings enables live sync
• On toggle-on, settings are loaded from account data immediately
• Local changes are debounced 2 s then uploaded to account data
• Echo-token loop prevention: each upload embeds a random token; when the homeserver echoes the event back the hook recognises it and skips re-applying (avoiding a feedback loop)
• Events arriving from other devices are applied and merge over local state
• Non-syncable keys are always taken from local state and never uploaded: usePushNotifications, useInAppNotifications, useSystemNotifications, pageZoom, isPeopleDrawer, isWidgetDrawer, memberSortFilterIndex, developerTools, settingsSyncEnabled

JSON export / import

Available regardless of whether sync is enabled. Export triggers a browser download; import opens a file picker and merges the selected file into current settings (same schema validation as the sync path).

Files changed

File	Purpose
src/types/matrix/accountData.ts	SableSettings = 'moe.sable.app.settings' enum value
src/app/state/settings.ts	settingsSyncEnabled: boolean (default false)
src/app/utils/settingsSync.ts	serialize / deserialize / export / import helpers
src/app/hooks/useSettingsSync.ts	side-effect hook + status atoms
src/app/pages/client/ClientNonUIFeatures.tsx	wire SettingsSyncFeature for session lifetime
src/app/features/settings/general/General.tsx	SettingsSyncSection UI
src/app/utils/settingsSync.test.ts	26 tests for pure utility functions
src/app/hooks/useSettingsSync.test.tsx	13 tests for hook behaviour (mount load, debounce, echo-token, remote updates)

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

AI assistance level

• None — No AI tools were used
• Research only — AI used for research, brainstorming, or understanding existing code; no generated code included
• AI-assisted — AI suggested code or logic that I reviewed, tested, and understand fully; I can explain every change
• Primarily AI-generated — The majority of code was AI-generated; I have thoroughly reviewed it, verified correctness, and made it my own

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

[github-actions]

Deploying with    Cloudflare Workers

Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-409-sable.raspy-dream-bb1d.workers.dev	1e07701	pr-409	Fri, 20 Mar 2026 03:26:32 GMT

[Just-Insane]

Tests well, can change settings and see them syncing to account data, uploading and downloading the files also works and overwrites settings as expected.

An example settings export file is below:
sable-settings-1773977439924.json