Chat motion: dedupe --chat-ease onto react-ui's real easing tokens

[TheGreatAxios]

Summary

• packages/chat-ui/src/styles.css invented its own --chat-ease cubic-bezier instead of consuming react-ui's --ease-out (CL-6487 finding Bench chat defaults with per-channel context-window overrides #3). It now aliases var(--ease-out, ...), keeping the old curve only as a fallback for contexts without react-ui's theme stylesheet loaded.
• The composer's auto-grow height transition (a resize-in-place, not an entrance) now uses --ease-in-out per DESIGN.md's explicit guidance, instead of the entrance curve every other --chat-ease transition in this file uses.
• chat-workbench-loading-tip-fade was animating at 400ms, over DESIGN.md's 150–300ms motion ceiling; dropped to 280ms.

Context / audit findings already resolved on main

Investigated CL-6487/6488/6489 (the audit tickets backing this lane) against current main. Three of CL-6488's four "chat canon" criticals are already fixed by a prior commit (3429637b, merged via PR #221) even though the Linear ticket is still marked Todo:

• Own messages are NOT right-aligned bubbles — .chat-bubble has no border/background/radius, and there is no row-reverse/[data-own] styling anywhere in the stylesheet.
• The composer is NOT capped at 42rem — it's width: 100% in .chat-composer-stack/.chat-composer.
• The tool-activity running/pending marker does NOT use brand --primary — it's var(--muted-foreground); only the failed state gets color, matching DESIGN.md's "color reserved for failure" rule.

Known gap (not fixed here — flagging honestly)

• Model attribution per turn (CL-6488 finding Approvals: show who and where, badge needs-you in the shell #2): the mock's .when slot shows "10:03 · opus-5" for agent turns. MessageSender (packages/chat-ui/src/api.ts) has no model field, and no model is persisted onto a message anywhere in packages/chat today (it's resolved per-turn for the LLM call in inference-preferences.ts/platform-adapter.ts but never attached to the posted message). Closing this needs a wire schema change plus orchestrator work in packages/chat, which is out of this lane's chat-ui-only scope and risks colliding with sibling lanes — flagging rather than rushing a partial fix.
• CL-6487's other findings (stale connection status polling, sidebar row-menu tap target, DESIGN.md/react-ui token reconciliation, .onboarding-phase-in 320ms) are outside the chat surface (Settings/Plugins/sidebar chrome) — left for the owning lane.
• CL-6489 (shell/onboarding canon violations) is entirely outside chat scope.

Test plan

• bun install in the fresh worktree (was missing @happy-dom/global-registrator), then bun test in packages/chat-ui: 650 pass / 0 fail
• bunx prettier --write on the changed file (no changes needed — already formatted)
• Manual read-through of timeline.tsx, tool-activity.ts/tool-activity-view.tsx, composer.tsx, chat-page.tsx, id-leak-guard.ts against mock-spec §12 — no further chat-scope gaps found beyond the model-attribution item above