revert(build): stop tracking the admin SPA build output

Tracking assets/admin/build was wrong — it is deploy-only output.
Revert to the ignored state; deploy by building on the target:
cd assets/admin && npm ci && npm run build

.gitignore

@@ -70,11 +70,12 @@ ask-*.md
 assets/css/routes/docs/*
 !assets/css/routes/docs/toolbar-search-enhancements.css
 
-# Installed dependencies (the SPA build output IS tracked so git-based deploys
-# carry assets/admin/build as one unit; rebuild with: cd assets/admin && npm run build)
+# Frontend build output and installed dependencies.
+# assets/admin/build is deploy-only: build it on the target with
+#   cd assets/admin && npm ci && npm run build
 /assets/admin/node_modules/
-/assets/admin/node_modules/*
-!/assets/admin/node_modules/.gitkeep
+/assets/admin/build/*
+!/assets/admin/build/.gitkeep
 assets/admin/.vite-temp/
 
 # PHP installed dependencies