Add POS background extension template

[vctrchu]

Background

Adds a scaffold for POS background extensions targeting pos.app.ready.data, the persistent background target shipping in the 2026-07 API. Resolves shop/issues-retail#29623.

Supersedes the earlier closed attempt (#408), which built the scaffold by overwriting the web-pixel-extension template. This lands it as its own template instead, leaving web-pixel-extension untouched.

Solution

• New pos-background/ template dir, mirroring the existing pos-action / pos-block / pos-smart-grid structure.
• Registered pos_background in templates.json (ui_extension, TypeScript + JavaScript flavors).
• Scaffold targets pos.app.ready.data with api_version = "2026-07" and example shopify.addEventListener handlers for transactioncomplete, cashtrackingsessionstart, and cashtrackingsessioncomplete.
• Dependency pinned to the published @shopify/ui-extensions 2026.7.x, matching sibling templates.

Checklist

• I have 🎩'd these changes
• I have squashed my commits into chunks of work with meaningful commit messages