feat: always show language selector on /start, add change_language button in profile

- /start now always shows language picker (removed language_set check)
- Added 'Change Language' button in profile inline keyboard
- Added handleChangeLanguage callback handler
- Added profile.change_language locale key in en/es/de
- Registered change_language callback route

src/i18n/locales/de.json

@@ -23,6 +23,7 @@
     "available_balance": "Verfügbares Guthaben",
     "member_since": "📅 Mitglied seit",
     "set_location": "📍 Standort festlegen",
+    "change_language": "🌐 Sprache ändern",
     "delete_account": "❌ Konto löschen",
     "error_loading": "Fehler beim Laden des Profils. Bitte versuche es erneut."
   },