Preview tools can't emulate prefers-reduced-motion

[juliusmarminge]

What happened

Agents can't emulate prefers-reduced-motion in a preview tab. preview_set_appearance only sets prefers-color-scheme, and the server browser applies only colorScheme through page.emulateMedia. So bugs that only appear with reduced motion can't be reproduced or verified with the preview tools.

Fleet evidence: 44 friction entries from 2026-10-24 to 10-26 ("Preview lacks reduced-motion emulation", "Required reduced-motion emulation unavailable"). The issue lead for https://git.cafe/pingdotgg/t3code/issues/24183 parked on it in 40 rounds.

Expected

preview_set_appearance (or a sibling option) accepts reducedMotion: "reduce" | "no-preference" | "system" and applies it on both the server browser (emulateMedia({ reducedMotion })) and the desktop browser (Emulation.setEmulatedMedia features).

Where (origin/main fe507c5a1e)

• apps/server/src/mcp/toolkits/preview/tools.ts:120: preview_set_appearance input is color scheme only.
• apps/server/src/preview/ServerBrowser.ts:642-653 applyRendering: emulateMedia({ colorScheme }) only.
• apps/desktop/src/preview/Manager.ts:2283, :2312: the desktop browser sends Emulation.setEmulatedMedia with prefers-color-scheme only.