[Feature] Add "Open in Shopify POS" CTA for POS extensions

[Easy-Ez]

WHY are these changes introduced?

When developing POS UI extensions, the Dev Console “View mobile” flow requires scanning a QR code every time. On mobile devices, we can deep link directly into the Shopify POS app using the existing
com.shopify.pos://... URL, reducing friction while keeping QR + copy as fallbacks.

Demo

97391bd935a0e306a62d6e72ba94c4ba.mp4

WHAT is this pull request doing?

• Adds a third call-to-action in the QR code modal for POS extensions: “Or open it directly in Shopify POS:” with an ExternalIcon.
• Shows the CTA only on mobile devices to avoid a misleading/invalid action on desktop browsers.
• Adds an isMobileDevice() utility and unit tests for the new rendering conditions.
• Uses flex-wrap on the CTA row to avoid the icon overflowing on narrow viewports.
• Adds a changeset for @shopify/ui-extensions-dev-console-app.

How to test your changes?

1. In a project with a POS UI extension, run shopify app dev and open the Dev Console.
2. In the Extensions list, click “View mobile” on a POS extension.
3. On a mobile device (or mobile emulation with a mobile UA), verify the third line appears: “Or open it directly in Shopify POS:” and the External icon button.
4. Tap the External icon; it should attempt to open Shopify POS via com.shopify.pos://....
5. Verify on desktop that the third CTA is not shown.
6. Run tests: pnpm -C packages/ui-extensions-dev-console vitest

Post-release steps

None.

Measuring impact

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[Easy-Ez]

I have signed the CLA!