refactor: let composing site own shell and brand CSS

Drops the re-bundled shell stylesheet from the app so lazy-loaded app
chunks no longer re-declare Paragon's :root tokens and clobber the
site's brand values. Moves the dev harness's shell load into a new
src/site.scss, removes the ./app.scss subpath export, and drops the
@edx/brand dependency entirely since the dev harness now runs
unbranded. App-scoped styles continue to load via Main.tsx.

Refs openedx/frontend-base#232

Co-Authored-By: Claude <noreply@anthropic.com>

Author: arbrandes

package-lock.json

@@ -10,8 +10,7 @@
     "url": "git+https://github.com/openedx/frontend-app-authn.git"
   },
   "exports": {
-    ".": "./dist/index.js",
-    "./app.scss": "./dist/app.scss"
+    ".": "./dist/index.js"
   },
   "files": [
     "/dist"
@@ -59,7 +58,6 @@
     "url": "https://github.com/openedx/frontend-app-authn/issues"
   },
   "dependencies": {
-    "@edx/brand": "npm:@openedx/brand-openedx@^1.2.3",
     "@edx/openedx-atlas": "^0.7.0",
     "@fortawesome/fontawesome-svg-core": "^6.7.2",
     "@fortawesome/free-brands-svg-icons": "^6.7.2",

package.json

@@ -2,7 +2,7 @@ import { EnvironmentTypes, SiteConfig } from '@openedx/frontend-base';
 
 import { authnApp } from './src';
 
-import './src/app.scss';
+import './site.scss';
 
 const siteConfig: SiteConfig = {
   siteId: 'authn-dev',

site.config.dev.tsx

@@ -0,0 +1,9 @@
+/*
+ * Site-level stylesheet for the standalone dev harness.  As the composing
+ * site here, the dev harness is the single owner of global styles and
+ * loads the shell stylesheet (which includes Paragon's base CSS) so that
+ * lazy-loaded app chunks don't re-inject `:root` custom properties.
+ *
+ * See frontend-base docs/how_tos/theming.md for the CSS ownership rationale.
+ */
+@use "@openedx/frontend-base/shell/app.scss";