fix(app): recover SSE after mobile app resume

[cmt-t]

Issue for this PR

Closes #39030

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

On iOS Safari, switching to another app can suspend the page and close the OpenCode SSE connection. When the user returns, the assistant may have finished on the server while the web UI remains stuck until a manual refresh.

This change reconnects the stream when the document becomes visible again. It also detects silently stalled streams and resynchronizes active session status and message timelines after reconnect. The implementation builds on the recovery work in #41299.

How did you verify your code works?

• Focused tests: 27 passed, 0 failed.
• bun run --cwd packages/app typecheck passed.
• Production validation: an ARM64 OpenCode build recovered an iOS Safari session after the page stayed in the background for an extended period, without a manual refresh.

Screenshots / recordings

No UI surface changed; this is a connection recovery behavior change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related Open PRs Found

#41299 - fix(app): auto-reconnect stale SSE streams and repair state drift

• This is directly referenced in the current PR's description as prior recovery work that this PR builds upon

#39349 - fix(app): recover from a silently dead event stream

• Related to detecting and handling stalled event streams, which is a core mechanism in PR fix(app): recover SSE after mobile app resume #44903

#43607 - fix(provider): ignore SSE comment heartbeats for chunk timeout

• Related to SSE heartbeat handling, which intersects with the heartbeat detection logic in PR fix(app): recover SSE after mobile app resume #44903

Note: PR #44903 is the current PR, so it's correctly excluded from the duplicates list. The other PRs are related but appear to be either prior work, complementary fixes, or addressing related aspects of SSE stream handling.

[github-actions]

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[github-actions]

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