feat(web): scaffold accelerated checkout buttons element

[rsomlette]

What

Adds a first-class Wallets view to the web playground (pnpm sample, /wallets.html) for testing <shopify-accelerated-checkout-buttons>, built with the same shell, settings panel, state, storage, and event-log patterns as the existing checkout demo.

Related: https://github.com/shop/issues-checkout/issues/14751

Settings (persisted in localStorage)

• Store: storefront domain, Storefront access token, country, language
• Purchase source toggle:
  • Existing cart — product grid + cart builder → "Create cart" calls cartCreate via the Storefront API and writes the returned GID to cart-id
  • Buy now — same product grid in selectable-card mode (click to pick a variant) → sets variant-id, optional selling-plan-id
• Options: wallet count (0 = all), layout (horizontal / vertical)

Center workspace

• Element mount with live attribute dump (always visible)
• Product grid (always visible, loaded from /products.json)
  • Cart mode: Add to cart / quantity controls / cart banner with lines + derived permalink
  • Buy now mode: selectable cards with selected indicator
• Cart banner (cart mode only) with "Create cart" button

Events panel

• Every settings change logs property writes + reflected attributes
• cart.creating / cart.created / cart.error events from the Storefront API call
• Future element events (wallets.render, wallets.error) wired in — names centralised in WALLETS_EVENT_TYPES
• New entries flash with an accent highlight animation (shared with the checkout demo)

Element scaffold changes

• Added layout attribute/property (horizontal default, vertical)
• Added log-level attribute/property (shared Logger from src/logger.ts)
• Extracted wallets.types.ts — WalletsAttributes, WalletsProperties, WalletsLayout interfaces (mirrors checkout.types.ts)
• Class now implements WalletsAttributes, WalletsProperties
• Shadow DOM uses createTemplate(html...) + cloneNode (matches checkout element)
• JSDoc @attribute annotations for the custom elements manifest
• Widened renderProducts / renderCart / selectGeneratedCartUrl / createProductLoader to accept interfaces so both demos share the same view code

New files

File	Purpose
src/wallets.types.ts	Attribute, property, and layout type interfaces
sample/storefront-api.ts	cartCreate Storefront API helper (testable, injectable fetcher)
sample/wallets-main.ts	Entry point
sample/wallets-state.ts	Store and state types
sample/wallets-storage.ts	localStorage persistence
sample/wallets-dom.ts	DOM refs
sample/wallets-shell.ts	HTML shell for tests
sample/wallets-render.ts	Top-level render
sample/views/wallets-settings.ts	Settings panel render
sample/views/wallets-element.ts	Element attribute dump render
sample/views/wallets-log.ts	Event log render
sample/views/log-entries.ts	Shared incremental log renderer with flash animation
+ .test.ts for each

Checks

• pnpm test — 353 tests pass
• pnpm lint — typecheck + sample:typecheck + oxlint + format ✅
• pnpm build + pnpm sample:build ✅
• No runtime dependencies added, no changes to the checkout demo behavior