Mobile browser tab does not reconnect SSE stream after returning from another app

Description

When accessing opencode serve or opencode web from a mobile browser (Chrome Android), switching to another app and coming back leaves the chat interface frozen. No new messages appear until the user manually refreshes the page.

Steps to reproduce

1. Run opencode serve --hostname 0.0.0.0
2. Open the web interface in Chrome on an Android phone
3. Start a conversation that generates a long response
4. Switch to another app (e.g. WhatsApp, settings, etc.)
5. Wait a few seconds
6. Switch back to Chrome

Expected behavior

The chat should show any new messages that arrived while the tab was backgrounded.

Actual behavior

The chat remains frozen at the last state before switching apps. Only a manual page refresh (F5 or pull-to-refresh) updates the interface.

Root cause

The SSE event stream in packages/app/src/context/server-sdk.tsx only handles tab lifecycle via pagehide/pageshow events, which are designed for bfcache (back-forward cache) navigation. On mobile:

1. pagehide often does not fire when a tab is backgrounded by switching apps
2. When it does fire, it calls stop() which aborts the SSE connection and may cancel server-side response generation
3. pageshow fires with event.persisted=false on return (not a bfcache restore), so the existing handler does nothing

The fix should listen to document.visibilitychange to reconnect the SSE stream when the tab becomes visible again.

Environment

• OpenCode version: 1.17.20+
• Device: Android phone (Chrome browser)
• Server: opencode serve on Linux