Migrate the asset pipeline from Sprockets to Propshaft

[superconductor-for-github]

Stacked on #527. Base branch is cycomachead/211-upgrade-css-compilation/1, so this diff shows only the Propshaft delta. Merge #527 first (see Before this can land below).

Follow-on to the Dart Sass migration in #527. Same compiler, different plumbing: Propshaft digests and serves what is on disk and does no transformation of its own, so Dart Sass writes app/assets/builds/application.css and Propshaft takes it from there.

sprockets-rails      ->  propshaft
dartsass-sprockets   ->  dartsass-rails

Propshaft has been the default asset pipeline for new Rails apps since Rails 8.

What goes away

• app/assets/config/manifest.js
• the config.assets.precompile list in config/initializers/assets.rb
• five Sprockets-only environment settings: assets.debug, assets.digest, assets.quiet, assets.compile, assets.css_compressor

Four things that needed handling

1. Font Awesome webfonts 404'd. font-awesome-sass emits font-path() only when Sprockets' Sass helpers are loaded, and otherwise falls back to #{$fa-font-path}/#{$file} — i.e. ../webfonts/…, which resolves to nothing. Fixed by setting $fa-font-path to the gem's logical asset path and letting Propshaft rewrite the url() to a digested one.

2. Gem Sass sources leaked into public/assets. Propshaft has a single load path and serves everything on it, and the bootstrap and font-awesome-sass gems put their Sass sources there so application.scss can @import them — so the first precompile digested 110 .scss partials into public/assets (11 MB → 9.2 MB once fixed).

They can't simply be excluded: dartsass-rails derives its --load-path list from config.assets.paths, and Propshaft's excluded_paths removes entries from that same array, so excluding them breaks the build. config/initializers/dartsass.rb therefore passes the two directories to Dart Sass explicitly as --load-path and then takes them off Propshaft's path. (font-awesome-sass/assets/fonts has to stay — that's where the webfonts live.)

3. The CSS is a build artifact now. A checkout without app/assets/builds/application.css raises Propshaft::MissingAssetError rather than rendering unstyled, which is the right failure mode but means:

• bin/setup builds it, so a fresh clone doesn't 500 on first page load
• Procfile.dev gains a css process (rails dartsass:watch), so make dev keeps it fresh
• the three workflows that invoke rspec/cucumber directly get a Build CSS step

4. Dev output regressed. dartsass-rails defaults to --style=compressed --no-source-map in every environment, where sassc-rails only compressed outside development. Development now builds expanded, with source maps pointing back at the .scss sources.

Verification

• rspec — 584 examples, 0 failures
• rspec --tag a11y — 40 examples, 0 failures, and all 40 screenshots pixel-compared against the Sprockets build: 20 byte-identical, the rest differing only where generated timestamps change column widths. No styling changes.
• cucumber — unchanged at the same 5 pre-existing failures (enrollments.feature, requests.feature; these fail on main too)
• Production boot: CSS, webfonts and all 27 local importmap assets return 200; no .scss in public/assets
• Development boot: 200, expanded CSS with a working digested source map
• Compiled CSS is token-for-token identical to the Sprockets output (14,377 tokens both) apart from minifier cosmetics — nested calc() and leading zeros. 351,895 vs 358,871 bytes.
• Checked .platform/.ebextensions for gzip_static: nothing there, so losing Sprockets' .gz files is a non-issue.

Relationship to #527

#527 has been rebased onto main and is now mergeable: clean. Resolving it kept the SCSS approach, deleted public/datatables/ and dropped the two <link> tags — and went further, per review: the vendored DataTables copies are gone entirely, replaced by @imports out of node_modules pinned to the same versions as the JavaScript in config/importmap.rb.

That means node_modules is a prerequisite for compiling CSS, and #527 already adds the Node setup to the Docker build stage, the three test workflows and bin/setup. This PR carries the dartsass-rails equivalent of that load-path config.

Merge #527 first; this one is stacked on it.

Please check

Blazer (/admin/blazer) and Faultline (/admin/errors). Both engines boot and their assets precompile, and Blazer 3.4 ships a Propshaft-aware layout branch that lists each file individually instead of relying on //= require — but I couldn't authenticate to click through either UI.