Centralize appearance preview events (codex)

[numnx]

🧩 Task: Centralize appearance preview events

Automated task PR opened by Code UX for T01.

📋 Summary

Sprint: Sprint 171 · Onboarding light mode · Task: T01

Fix onboarding appearance so light mode and background choices render immediately inside the onboarding experience, with shared preview behavior, regression coverage, and updated docs.

🤖 Model & Provider

Provider	Codex CLI
Model	gpt-5.5

⏱️ Timing

Started	2026-07-07 09:32:43 UTC
Finished	2026-07-07 09:39:01 UTC
Duration	6m 18s

📊 Token Usage

Metric	Value
Input tokens	209,378
Cached input tokens	747,776
Output tokens	6,430
Total tokens	963,584
Invocations	1
Tool calls	34
Active time	5m 31s

Included usage estimate (subscription/local login): $1.61

✅ QA Review

⏳ QA review runs after this PR is opened. Check the task card in the dashboard for the outcome once it lands.

📝 Full Task Prompt

## Objective
Add a small shared helper for the existing `codeux:appearance-preview` contract so Settings and Onboarding can publish unsaved appearance state consistently.

## Scope
- Create `dashboard/src/v2/lib/appearance-preview.ts`
- Create `dashboard/src/v2/lib/__tests__/appearance-preview.test.ts`
- Edit `dashboard/src/v2/hooks/use-settings-page-state.ts`
- Verify existing `tests/dashboard/v2/settings-page-state.test.tsx` preview behavior
- Relevant symbols: `codeux:appearance-preview`, `DashboardSettings["appearance"]`, `useSettingsPageState`

## Implementation Requirements
1. Add exported helpers such as `publishAppearancePreview(appearance: DashboardSettings["appearance"] | null): void` and, if useful, `clearAppearancePreview(): void`; helpers must no-op when `window` is unavailable.
2. Move direct `window.dispatchEvent(new CustomEvent("codeux:appearance-preview", ...))` usage in `use-settings-page-state.ts` to the new helper while preserving the existing event detail shape `{ appearance }`.
3. Add focused tests for publishing a non-null appearance payload, publishing `null`, and SSR/no-window safety if the test environment can exercise it without brittle globals.
4. Keep the existing settings-page preview lifecycle unchanged: editable appearance changes publish previews, and unmount publishes `appearance: null`.

## Constraints
- Do not change `codeux:settings-updated`; this task only touches unsaved appearance preview events.
- Do not alter settings save/load APIs or persisted schema.
- Preserve `tests/dashboard/v2/settings-page-state.test.tsx` expectations for preview event payloads and cleanup.
- Keep the helper free of Preact hooks so it can be used from components and hooks.

## Verification
- Run `pnpm exec vitest run dashboard/src/v2/lib/__tests__/appearance-preview.test.ts tests/dashboard/v2/settings-page-state.test.tsx`.
- Run `pnpm run typecheck:dashboard`.
- Success means the new helper emits the same event detail shape and existing Settings preview tests still pass.

🌿 Branch Info

Base: feature/CODUX-171-onboarding-light-mode
Head: task/feature-codux-171-onboarding-light-mode-t01-codex-mragankr

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🤖 Generated by Code UX