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.

biome.json

@@ -7,7 +7,10 @@
       "!app/public/assets/js/htmx.min.js",
       "!app/public/assets/js/overtype.min.js",
       "!app/public/assets/js/sortable.min.js",
-      "!app/public/assets/js/vendor/**"
+      "!app/public/assets/js/vendor/**",
+      "!app/public/assets/css/filepond*.css",
+      "!app/public/assets/css/modern-normalize*.css",
+      "!app/public/assets/dist/**"
     ]
   },
   "css": {
@@ -15,7 +18,7 @@
       "enabled": true
     },
     "linter": {
-      "enabled": false
+      "enabled": true
     }
   },
   "linter": {