feat: add Polish (pl) locale

Full Polish translation covering all UI modules — tasks, calendar,
shopping, meals, budget, notes, contacts, birthdays, recipes,
documents, housekeeping, and settings. Polish is now selectable in
Settings → Language.

Bumps to v0.50.0.

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

public/components/oikos-locale-picker.js

@@ -23,6 +23,7 @@ const LOCALE_LABELS = {
   hi: 'हिन्दी',
   pt: 'Português',
   uk: 'Українська',
+  pl: 'Polski',
 };
 
 class OikosLocalePicker extends HTMLElement {

public/i18n.js

@@ -5,7 +5,7 @@
  * Dependencies: none (vanilla JS, Fetch API, Intl API)
  */
 
-const SUPPORTED_LOCALES = ['de', 'en', 'es', 'fr', 'it', 'sv', 'el', 'ru', 'tr', 'zh', 'ja', 'ar', 'hi', 'pt', 'uk'];
+const SUPPORTED_LOCALES = ['de', 'en', 'es', 'fr', 'it', 'sv', 'el', 'ru', 'tr', 'zh', 'ja', 'ar', 'hi', 'pt', 'uk', 'pl'];
 const DEFAULT_LOCALE = 'de';
 const STORAGE_KEY = 'oikos-locale';
 const DATE_FORMAT_KEY = 'oikos-date-format';