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).

src/SlpModularCms.Api/Extensions/WebsitePlaceholder.html

@@ -20,7 +20,7 @@
         }
         @media (prefers-color-scheme: dark) {
             body { background: #141414; color: #ededed; }
-            code { background: #262626; }
+            code { background: #262626; color: #141414; }
             a { color: #ff6b6b; }
         }
         main { max-width: 34rem; }
@@ -28,6 +28,7 @@
         p { margin: 0 0 1rem; }
         code {
             background: #ececec;
+            color: #1a1a1a;
             padding: 0.15em 0.4em;
             border-radius: 4px;
             font-family: ui-monospace, "Cascadia Code", Consolas, monospace;