chore: sync react router template with example

[fredericoo]

TL;DR: Refreshes the standalone React Router template from the current example and keeps it deployable to Oxygen without monorepo dependencies.

Before

templates/react-router had drifted from examples/react-router and was missing newer storefront routes, cart behaviour, predictive search, Customer Accounts, and SEO resource routes.

After

The template now uses the current React Router example as its source, with an Oxygen Worker runtime. The source template stays on the workspace (workspace:*, pnpm, no lockfile); the release flow pins the @shopify/hydrogen preview version and generates an npm lockfile in the distributed copy only.

What this changes

• Refreshes the storefront UI and routes from examples/react-router.
• Adds Customer Accounts, predictive search, sitemap, and robots routes.
• Replaces workspace/shared imports with template-local modules.
• Uses Mini Oxygen with Worker env, cache, and waitUntil context.
• Updates the template README and the example-to-template conversion guidance.

Developer impact

No changeset. This updates a standalone preview template and repo-local conversion documentation; it does not change a published package API.

UX impact

The template gains the newer product, collection, search, cart, account, consent, and accessibility behaviour from the React Router example.

Risk

• Oxygen runtime wiring, Customer Account session handling, and cart flows deserve focused review.
• Customer Accounts require a real store, HTTPS, and the documented environment values.

How to Test

1. Run pnpm install from the repository root.
2. Run pnpm dev in templates/react-router with no .env file and open http://localhost:5173.
3. Confirm the home, collections, search, cart, robots, and sitemap routes render against mock.shop.
4. Run pnpm preview in templates/react-router and confirm the same routes render through the built Mini Oxygen Worker.
5. Optionally configure the real-store and Customer Account environment values from .env.example, deploy to Oxygen, and verify account login/logout.

[andguy95]

Review with Freddie