build(admin): track the SPA build output so git deploys carry it

assets/admin/build was gitignored, so servers deployed by git pull kept
running a stale admin bundle — the icon picker integration in the tabbar
and floating-button settings pages never reached them. Track the build
directory (rebuild with: cd assets/admin && npm run build) and commit the
current bundle containing the universal icon picker.

.gitignore

@@ -70,9 +70,9 @@ ask-*.md
 assets/css/routes/docs/*
 !assets/css/routes/docs/toolbar-search-enhancements.css
 
-# Frontend build output and installed dependencies
-/assets/admin/build/*
-!/assets/admin/build/.gitkeep
+# 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)
+/assets/admin/node_modules/
 /assets/admin/node_modules/*
 !/assets/admin/node_modules/.gitkeep
 assets/admin/.vite-temp/