Sync environment settings across connected devices automatically

[prdai]

Problem

Settings that T3 stores per environment do not follow a user across machines. Changing a setting on one server only affects that server unless the Settings scope selector is used to apply the change to "All environments" manually. That selector is a one-shot bulk edit, not a sync: a later change on one machine does not reach the others, and a machine that is offline during the edit keeps the old value.

The same split shows up elsewhere:

• Appearance preferences are saved separately on each device or browser (docs/user/appearance.md).
• General, Appearance, Chat, Diffs, SnapShot, Connections, Access, and About are marked device-only in SETTINGS_DEVICE_ONLY_PATHS (apps/web/src/components/settings/SettingsScopeSentence.tsx).
• Mobile client settings have no client-settings sync at all (Working section beta toggle on desktop does not carry over to the iPhone app #19094).
• Desktop settings not applying to a remote environment were reported in [Bug]: Settings changes in the desktop app don't apply to a remote environment #7500.

The result: the same account on a desktop, a laptop, and a remote server drifts, and every change has to be re-applied per machine.

Expected

One of:

1. Environment-scoped settings sync automatically between all connected environments under the same account, with an opt-out per setting.
2. An explicit "sync across environments" control per setting, so a value chosen once stays applied when a new environment connects.
3. Settings that are intentionally device-local stay local, and the UI says so on each affected page.

Ask

Confirm whether automatic sync of server-side settings across environments is planned. If it is not, label the device-only settings in the UI so users stop looking for a sync toggle that does not exist.

[xkjjx]

A concrete case from a MacBook plus a Mac Mini, both connected to the same client, with snooze options.

What happened

I edited Settings → Threads → Snooze options with All projects across All environments selected. The new list reached the MacBook but not the Mac Mini. Since snooze menus use the presets of the environment each thread lives on, the sidebar ended up with two different snooze menus: Mac Mini threads offered the built-in defaults, and MacBook threads offered my list.

Why it was hard to figure out

• The editor shows one environment's list. It shows the MacBook's presets with no Mixed state, even though docs/user/project-settings.md says a control shows Mixed when the selected environments disagree. The only persistent signal is the amber layers icon.
• The source popover can't show the difference. Both environments read "5 items" (formatValue summarizes arrays by length). The MacBook's comes from its Environment layer and the Mac Mini's from Default, so the lists look identical even though their contents differ.
• The cause only shows up as a toast on save. The Mac Mini was on an older build that predates snoozePresets, so it accepted the write and dropped the key. Nothing on the page says an environment is outdated. The toast is the only explanation, and after it disappears, the editor keeps showing the saved list as if every environment had it:

What would have made this smooth

1. Flag outdated environments in place. When a selected environment is too old for a setting, say so on that row (or in the scope selector), with a link to update it, before the user tries to save.
2. Offer to reconcile on reconnect or update. When an environment reconnects, or finishes updating, and its shared settings (SHARED_SERVER_SETTING_KEYS) differ from the others, prompt: "Mac Mini has different snooze options. Use these / Keep theirs." This is a smaller step than full automatic sync and covers both the offline case and the outdated-version case. It's close to option 2 in the original post.
3. Show Mixed for list settings. Snooze options should show the same Mixed state as other controls, and the source popover should show which items differ rather than only the count.

Related: the bulk-snooze menu in apps/web/src/components/Sidebar.tsx uses the presets of whichever selected thread comes first, with the comment "Presets sync across environments, so the first selected thread's stand for all." Until settings actually sync, bulk-snoozing threads from both machines uses whichever machine's list happens to come first.