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/en.json

@@ -23,6 +23,7 @@
     "available_balance": "Available Balance",
     "member_since": "📅 Member since",
     "set_location": "📍 Set Location",
+    "change_language": "🌐 Change Language",
     "delete_account": "❌ Delete Account",
     "error_loading": "Error loading profile. Please try again."
   },