Add subcategory management to admin panel + Kala/Liha defaults

Categories now show their subcategories as tags with remove buttons,
plus an input field to add new subcategories directly from admin.
Added Kala and Liha as default subcategories for Reseptit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api.php

@@ -104,6 +104,8 @@ function defaultCategories(): array {
          'subcategories' => [
              ['id' => 'kasvis',     'fi' => 'Kasvis'],
              ['id' => 'vegaaniset', 'fi' => 'Vegaaniset'],
+             ['id' => 'kala',       'fi' => 'Kala'],
+             ['id' => 'liha',       'fi' => 'Liha'],
              ['id' => 'jalkiruuat', 'fi' => 'Jälkiruuat'],
              ['id' => 'muut',       'fi' => 'Muut'],
          ]],