Add Slovak (sk) language to the language selector

The Slovak translation file (client/strings/sk.json) already exists
with a complete translation but was missing from the languageCodeMap
in i18n.js, making it inaccessible from the language dropdown.

Also adds Slovakia to the podcast search region map.

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

client/plugins/i18n.js

@@ -28,6 +28,7 @@ const languageCodeMap = {
   pl: { label: 'Polski', dateFnsLocale: 'pl' },
   'pt-br': { label: 'Português (Brasil)', dateFnsLocale: 'ptBR' },
   ru: { label: 'Русский', dateFnsLocale: 'ru' },
+  sk: { label: 'Slovenčina', dateFnsLocale: 'sk' },
   sl: { label: 'Slovenščina', dateFnsLocale: 'sl' },
   sv: { label: 'Svenska', dateFnsLocale: 'sv' },
   tr: { label: 'Türkçe', dateFnsLocale: 'tr' },
@@ -67,6 +68,7 @@ const podcastSearchRegionMap = {
   pt: { label: 'Portugal' },
   ru: { label: 'Россия' },
   ch: { label: 'Schweiz / Suisse / Svizzera' },
+  sk: { label: 'Slovensko' },
   se: { label: 'Sverige' },
   vn: { label: 'Việt Nam' },
   ua: { label: 'Україна' },