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.

justfile

@@ -74,6 +74,10 @@ build-js:
 build-install:
     @npm ci
 
+[group('build')]
+build-lint:
+    @npx biome lint app/public/assets/css/ app/public/assets/js/app/ scripts/
+
 [group('build')]
 build-check:
     @echo "Checking if build output is up to date…"