[WiP] Add/site selector on design your own button

[xavier-lc]

Related to https://github.com/Automattic/dotcom-forge/issues/5386

Proposed Changes

• Show the site selector (introduced on https://github.com/Automattic/wp-calypso/pull/87260) when the user clicks on the "Design your own" button on the Theme Showcase if they haven't selected any site.

Testing Instructions

• Use a user with multiple sites.
• Enter the Theme Showcase page without site fragment (/themes).
• Click on "Select a site to activate".
• Check that the site selection modal appears correctly.
• Select a site.
• Check that the page is refreshed with the site fragment, and that the primary CTA button is updated accordingly (e.g. "Activate this design").
• Check that there's a notice that says "You have selected the site [SITE TITLE]."
• Check that clicking on the notice's "Choose a different site" reopens the site selector modal
• Try the same on themes of different tiers (e.g. Premium, Partner, Community).
• Ensure both the theme's primary CTA and any upsell nudges open the Site Selector modal instead of sending to a with-theme flow.

Pre-merge Checklist

• [ ] Has the general commit checklist been followed? (PCYsg-hS-p2)
• [ ] https://wpcalypso.wordpress.com/devdocs/docs/testing/index.md for your changes?
• [ ] Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• [ ] Have you checked for TypeScript, React or other console errors?
• [ ] Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• [ ] Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• [ ] For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions[bot]]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-99286

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-99286&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~602 bytes added 📈 [gzipped])

name    parsed_size           gzip_size
themes      +3310 B  (+0.3%)     +918 B  (+0.3%)
theme        +373 B  (+0.1%)      +94 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory. Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[a8ci18n]

This Pull Request is now available for translation here: https://translate.wordpress.com/deliverables/11510576

Some locales (Hebrew, Japanese) have been temporarily machine-translated due to translator availability. All other translations are usually ready within a few days. Untranslated and machine-translated strings will be sent for translation next Monday and are expected to be completed by the following Friday.

Thank you @xavier-lc for including a screenshot in the description! This is really helpful for our translators.

[a8ci18n]

Translation for this Pull Request has now been finished.