Opt-in "Stay connected" for iOS: keep environment connections open in the background

[aididhaiqal]

Problem

iOS suspends T3 Code a few seconds after you leave it, which closes every environment socket. When you come back, the app has to reconnect and catch up before threads are current. If you check on agents often, that's a delay on every switch. Push notifications and Live Activities cover "tell me when it's done", but:

• they need T3 Connect, so direct and Tailscale users have nothing while the app is in the background;
• they show summaries, not the live thread.

Terminal apps such as Termius and Blink solve the same problem with an opt-in background keep-alive.

Proposal

An iOS-only setting, off by default, at Settings → Stay connected:

• While it's on, the app runs a standard Core Location session at 3 km accuracy with the location background mode. iOS keeps the app running, so the existing WebSocket runtime stays connected and keeps renewing its activity lease. Nothing in the connection stack changes.
• It needs While Using the App location access, not Always. Updates start while the app is in the foreground and continue in the background with the system location indicator showing.
• Location readings are discarded right away. They are never stored or sent anywhere.
• Turning it off stops the session. Revoking location access in system Settings stops it too, and the screen shows how to turn access back on.

The prototype is about 100 lines of Swift and one settings screen. It adds no server or contract changes.

Trade-offs to decide

• Battery: coarse location is cheap, but keeping the app alive keeps the JS runtime and sockets active. The setting says so.
• App Review: the location background mode needs a written justification. Termius and Blink ship this pattern, but approval is not guaranteed. We would not use silent audio.
• Privacy perception: a coding app asking for location access needs clear copy. The permission prompt explains it's only used to stay awake.
• Android: not included. The equivalent there is a foreground service, which feat(mobile): keep Android connections alive in the background #5179 attempted and which was closed for missing approval. Android could be a separate decision.
• Alternative: a much smaller fix is beginBackgroundTask, which keeps sockets alive for about 30 seconds after you leave the app. It covers quick app switches but not "check back in 10 minutes".

Question for maintainers

Is an opt-in, iOS-only location-based keep-alive a direction you'd accept? If so, is the scope above right: off by default, While Using access only, no Android?

[aididhaiqal]

Prototype for reference (no PR until the direction is approved): aididhaiqal/t3code@main...feat/mobile-background-keep-alive

Related: #21134 fixes a separate Live Activity bug where a rotated per-activity push token was never registered, so cards stopped updating until the app was opened.