fix(web): preserve snapshot preview size in sent messages

[rynfar]

Summary

Ports upstream commit `a43f9b45ae85caf37e0be8270ad3d27365ece2bd` (upstream PR #11429), part of upstream integration cycle #536.

In `MessagesTimeline.tsx`, user message image attachments render in a 2-column grid (`grid grid-cols-2 max-w-[210px] gap-2`). Snapshot attachment frames (`SNAP_SHOT_ATTACHMENT_FRAME_CLASS`) have a fixed width of `w-52` (208px), which caused them to be squished horizontally into a single column. Applying `col-span-2` when the snapshot preview resolves allows the snapshot frame to span both columns of the grid container as intended.

Also advances the review cursor in `.agents/upstream-review.md` to `a43f9b45ae85caf37e0be8270ad3d27365ece2bd` (`2026-09-13`), accounting for all 3 sources in the range `d1d15c67f4..a43f9b45ae`:

• `18d8cbfd920d0a53e5b5206456585aea767e852c`: already merged in PR fix(release): pin expo-audio to its patched version #523
• `5349522108bec896bb6a69ae76c6e1b8d419ae16`: covered (unused `.pnpm-store` cleanup, absent in Pylon)
• `a43f9b45ae85caf37e0be8270ad3d27365ece2bd`: adopted in this PR

Fixes #536

Adversarial review

• Mixed attachments: If a snapshot attachment is accompanied by regular image attachments, the snapshot attachment correctly spans the full row (`col-span-2`), while subsequent image attachments tile 2-per-row below it.
• Unresolved preview URL: If `previewUrl` is not yet available, `col-span-2` is not applied and the fallback thumbnail box maintains standard `aspect-[4/3]` aspect ratio.
• Unit test coverage: `MessagesTimeline.test.tsx` verifies that `col-span-2` is present when a snapshot preview URL is resolved, and absent when unresolved. All 58 unit tests in `MessagesTimeline.test.tsx` pass.

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Project	Deployment	Actions	Updated
pylon-marketing	Ready	Preview	Sep 14, 2026 1:45am UTC

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	14.1 KiB	13.9 KiB	−180 B (−1.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	−8 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.9 KiB	6.7 KiB	−172 B (−2.4%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.8 KiB	58.0 KiB	−910 B (−1.5%)	66.4 KiB	✅
Codex	Live turn messages	10	8	−2 (−20.0%)	21	✅
Claude	Total thread wire	14.1 KiB	14.1 KiB	+36 B (+0.3%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	−5 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.8 KiB	6.9 KiB	+41 B (+0.6%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	59.7 KiB	59.7 KiB	+44 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	9	10	+1 (+11.1%)	21	✅

Baseline: 42c65f9 · PR result: 34fdbcc · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 115.6 KiB
• Claude decoded thread snapshot: 116.3 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.