Fix biome lint errors: remove duplicate CSS properties, apply safe auto-fixes

CSS:
- Remove duplicate 'background' fallbacks in base.css, header.css, search.css
  (solid color declared before gradient — gradient always wins)
- Remove duplicate 'padding' in admin.css .admin-import-log

JS (biome --write safe fixes applied):
- function() → arrow functions in all IIFEs and callbacks
- forEach/callback → arrow functions
- evaluePtrn → parseInt(x, 10) in admin-contacts-form.js
- Cleaned label text in build.mjs lint step

Remaining warnings are intentional: !important overrides, descending
specificity (admin.css cascade), noUnusedVariables (functions exported
to window/onclick), useTemplate style preference.

scripts/build.mjs

@@ -37,6 +37,10 @@ if (!existsSync(resolve(root, "node_modules"))) {
   execSync("npm ci", { cwd: root, stdio: "inherit" });
 }
 
+if (buildAll || onlyCss) {
+  run("Linting CSS + JS (biome)", "npx biome lint app/public/assets/css/ app/public/assets/js/app/ scripts/ || true");
+}
+
 if (buildAll || onlyCss) {
   run("Building CSS bundles", "node scripts/build-css.mjs");
 }