fix(web): restore the ultrawork chat replay on mobile — the right rail was starving it to 0px

[code-yeongyu]

Supersedes #104 (auto-closed by the source-guidance bot before the branch was rebased onto the #103 squash).

What broke

On every phone (≤768px), the landing's product anchor — the Codex-window chat replay — was invisible. The fixed 560px window rendered ONLY the Environment/Subagents right rail, with the footer's spinner and run-progress bar ghosting over it.

Why

The mobile media query set the window frame to a single grid column but left the rows implicit. The 696px-tall right rail and the transcript main pane became two auto rows inside a 558px box: the rail took everything, the transcript row computed to 0px (grid-template-rows: 0px 696.25px), and its children painted transparently over the rail.

Fix

• Hide .ulw-side at ≤768px — placed in ulw-demo-panel.css (which imports after ulw-demo-app.css, so the override wins the cascade) alongside the rail's own rules, mirroring the already-hidden session sidebar. With one grid child left, the auto row stretches and the transcript owns the fixed box.
• Strengthened the mobile e2e test to pin visibility, not attachment: the ask bubble must be visible and the transcript must occupy >250px of the window. RED against the broken layout (the old test counted DOM nodes, which kept passing while the replay was invisible), GREEN after.
• DESIGN.md layout-stability contract updated to document the rail behavior.

Verification

• RED → GREEN: .omo/evidence/v12-mobile-red.txt → v12-mobile-green.txt (4/4)
• Full sweep 57/57 (v12-full-e2e.txt), Lighthouse 100×4 (v12-lighthouse.txt)
• Before/after captures at 375/390/768 plus a full per-section mobile audit in .omo/evidence/v12-mobile/ — two independent visual-QA reviewers returned PASS with zero blocking findings.

Repo-local packages/web work, owner-directed — merging immediately (same source-guidance-bot situation as #103).

🤖 Generated with Claude Code

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Summary by cubic

Fixes the invisible chat replay on phones by hiding the right rail at ≤768px so the transcript fills the fixed window. Also tightens the mobile e2e to assert visibility and updates the layout contract.

• Bug Fixes
  • Hide .ulw-side at ≤768px in packages/web/app/styles/ulw-demo-panel.css so the grid no longer starves the transcript; the transcript owns the 560px window.
  • Strengthen packages/web/e2e/ulw-demo.spec.ts to require the rail is hidden, the first ask is visible, and the transcript height >250px.
  • Update packages/web/DESIGN.md to document that both the session sidebar and right rail hide on mobile.

^{Written for commit 7377d65. Summary will update on new commits.}