Modernize React Router template runtime

[fredericoo]

TL;DR: Establishes the current Oxygen and React Router runtime foundation before the discovery and purchase-flow template updates. This follows #3961.

Before

The template mixed legacy context fields with Router Context, did not provide Oxygen cache/waitUntil to the Storefront client, used static shop identity, and had no Customer Account route or persistent account session.

After

Middleware owns the Hydrogen request lifecycle using request-scoped Storefront and Customer Account clients, typed Worker contexts, Oxygen cache, encrypted account sessions, dynamic shop identity, and buyer-safe error handling.

What this changes

• Wires Worker env, cache, and waitUntil through Router Context.
• Adds fail-closed real-store configuration with zero-config mock.shop fallback.
• Adds Customer Account OAuth handlers, encrypted sessions, and /account.
• Sources the shop name from Storefront API data and uses one static Collections navigation link.
• Keeps Shopify Scripts, consent, and global analytics identity aligned with the active store.
• Uses Hydrogen's automatic local HTTPS certificate provisioning for Customer Account development.
• Updates the template README and Oxygen-template maintainer guidance.

Developer impact

No changeset. This updates the standalone React Router template and repo-local maintainer guidance without changing a published package API.

UX impact

The storefront displays its real shop name, provides a working Account destination, and retains the existing visual system. Customer Accounts remain disabled with setup guidance in mock mode.

Out of scope

• Collection, search, or predictive-search modernisation.
• Cart and product-detail modernisation.
• Visual-system replacement and SEO resource routes.

Risk

• Middleware response finalisation and request-body handling affect every route.
• Customer Account OAuth depends on exact HTTPS callback configuration.
• Real-store identity now fails closed instead of mixing configured credentials with demo defaults.

How to Test

1. Run pnpm --filter @shopify/hydrogen build.
2. Run pnpm --filter @shopify/hydrogen-template-react-router dev.
3. Open /, /collections, /search, /cart, and /account with no .env; confirm mock.shop content renders.
4. Run pnpm --filter @shopify/hydrogen-template-react-router preview and repeat the route checks.
5. Configure the real-store and Customer Account values from .env.example, then run npm run https:dev and accept the first-run certificate prompt.
6. Register the printed callback, JavaScript-origin, and logout values in Shopify admin; verify login and logout.