Fixes /admin 404 by building the admin SPA before dotnet publish, not via an MSBuild target

The BuildAndCopyAdminFrontend MSBuild target (BeforeTargets=Publish) never actually
worked: files it created after project evaluation were silently absent from the
publish output, and forcing them in via an explicit Content item collided with the
SDK's own static-web-asset resolution. Verified locally with a clean obj/bin and a
fresh node_modules - every deploy so far genuinely shipped without wwwroot/admin.
Moves the frontend build + copy into its own CI step ahead of dotnet publish, so the
SDK's ordinary wwwroot handling picks it up with no custom MSBuild involved.

Also fixes unreadable <code> badges on the website placeholder page (no explicit
text color, relying on inherited body color pairing unreliably with the badge
background).

.gitea/workflows/continuous_integration.yaml

@@ -253,13 +253,29 @@ jobs:
             exit 1
           fi
 
-      - name: Publish (test)
-        working-directory: src/SlpModularCms.Api
+      # Builds the admin SPA and lands it in wwwroot/admin *before* dotnet publish runs, as its own
+      # step rather than an MSBuild target hooked to Build/Publish. That was tried and reliably
+      # failed two different ways (see SlpModularCms.Api.csproj's comment on the topic): the SDK's
+      # wwwroot static-web-asset item set is fixed at project evaluation time, before any target
+      # runs, so files a target creates afterward never make it into the publish output; and forcing
+      # them in as an explicit Content item collided with the SDK's own static-web-asset resolution.
+      # Physically existing on disk before dotnet publish/build ever runs is the only thing that
+      # worked, verified locally against a clean obj/bin and a fresh node_modules.
+      - name: Build admin frontend
+        working-directory: frontend
         env:
           VITE_APP_ENV: test
           VITE_UMAMI_SCRIPT_URL: ${{ vars.VITE_UMAMI_SCRIPT_URL }}
           VITE_UMAMI_WEBSITE_ID: ${{ vars.VITE_UMAMI_WEBSITE_ID_TEST }}
           VITE_SENTRY_DSN: ${{ vars.VITE_SENTRY_DSN }}
+        run: |
+          pnpm install --frozen-lockfile
+          pnpm build
+          mkdir -p ../src/SlpModularCms.Api/wwwroot/admin
+          cp -r dist/. ../src/SlpModularCms.Api/wwwroot/admin/
+
+      - name: Publish (test)
+        working-directory: src/SlpModularCms.Api
         run: >
           dotnet publish -c Release -r ${{ env.PUBLISH_RID }} --self-contained false
           -o ${{ github.workspace }}/${{ env.ARTIFACT_NAME_TEST }}
@@ -300,13 +316,23 @@ jobs:
             exit 1
           fi
 
-      - name: Publish (production)
-        working-directory: src/SlpModularCms.Api
+      # See publish-test's "Build admin frontend" step for why this runs as its own step ahead of
+      # dotnet publish rather than an MSBuild target.
+      - name: Build admin frontend
+        working-directory: frontend
         env:
           VITE_APP_ENV: production
           VITE_UMAMI_SCRIPT_URL: ${{ vars.VITE_UMAMI_SCRIPT_URL }}
           VITE_UMAMI_WEBSITE_ID: ${{ vars.VITE_UMAMI_WEBSITE_ID_PRODUCTION }}
           VITE_SENTRY_DSN: ${{ vars.VITE_SENTRY_DSN }}
+        run: |
+          pnpm install --frozen-lockfile
+          pnpm build
+          mkdir -p ../src/SlpModularCms.Api/wwwroot/admin
+          cp -r dist/. ../src/SlpModularCms.Api/wwwroot/admin/
+
+      - name: Publish (production)
+        working-directory: src/SlpModularCms.Api
         run: >
           dotnet publish -c Release -r ${{ env.PUBLISH_RID }} --self-contained false
           -o ${{ github.workspace }}/${{ env.ARTIFACT_NAME_PRODUCTION }}