[markokajzer]

Summary

Rails 8 defaults to Propshaft, but solidus_backend only works with Sprockets: its JS and SCSS rely on Sprockets directives, sassc-rails, handlebars_assets and ERB, and Propshaft and sprockets-rails can't run side by side. This follows solidusio#6404 (Sprockets only belongs to the legacy backend) and the Rails main work in solidusio#6677 / solidusio#6679.

Nothing changes for Sprockets apps, and nothing is prebuilt: the host app still compiles everything.

• Stylesheets compile under both sassc and dart-sass: plain url() instead of image-url/font-url (Sprockets still rewrites them to digested paths), themes renamed from .css.scss to .scss, and Font Awesome 4.7's SCSS vendored because font-awesome-rails only ships a .css.erb
• Propshaft apps add dartsass-rails: the backend registers its themes as dartsass builds, so assets:precompile and dartsass:watch compile them with the app's own stylesheets
• JavaScript needs no build step: under Propshaft, Spree::Backend::PropshaftManifest expands the //= require directives (including the app's spree/backend/all.js) into individual script tags, in the order Sprockets would have concatenated them. Handlebars templates are compiled in the browser, and spree.js.erb is rendered inline
• solidus_core only requires sprockets/railtie when it's installed and Propshaft isn't loaded; solidus_backend skips sassc-rails under Propshaft

CI runs the backend specs on Propshaft for the Rails 8.1 / Ruby 4.0 row.

Verification

• 107 order, customer, new order, taxon, stock management, customer return, product edit, homepage, locale and style guide feature examples pass under both Sprockets and Propshaft
• Screenshots of orders, order edit, customer, products, taxonomy and style guide match main under both pipelines (style guide and taxonomy pixel-identical; the rest differ only in random order/SKU numbers)

Alternatives considered

• Precompiling the backend's assets into the gem: a Node build (dart-sass, Handlebars precompile, directive resolver) writing plain JS/CSS under the same logical paths, which works with either pipeline without extra gems. Rejected because it adds a build step to Solidus's own development and release, and built output either has to be committed or is missing for apps that install from git.

Open questions

• Propshaft apps still install sassc-rails (and with it sprockets-rails and sassc) because solidus_backend depends on them; they just aren't loaded. Make them optional?
• The admin layout now loads around 130 individual files under Propshaft. That's the Propshaft way, but could move to importmap later.
• The install generator still writes Sprockets-style all.js / all.css. They work under Propshaft (their directives are expanded too), but could become plain files for new Propshaft apps.
• dart-sass deprecates @import. That's fine for now, but the backend's stylesheets will eventually need @use.

Checklist

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.