Implement username change functionality and enhance Telegram bot registration flow

- Added a new endpoint for changing usernames, allowing users to update their login credentials via the API.
- Integrated username change functionality into the settings page, providing a user-friendly interface for this action.
- Enhanced the Telegram bot to support user registration directly through the bot, including username generation and password delivery.
- Updated documentation to reflect the new username change endpoint and registration flow through the Telegram bot.

frontend/src/routes/settings.tsx

@@ -2,6 +2,7 @@ import { createFileRoute } from '@tanstack/react-router'
 import { useTranslation } from 'react-i18next'
 import { useRequireAuth } from '@/features/auth/guards'
 import { ChangePasswordForm } from '@/features/settings/ChangePasswordForm'
+import { ChangeUserNameForm } from '@/features/settings/ChangeUserNameForm'
 import { TelegramLinkCard } from '@/features/settings/TelegramLinkCard'
 import { DeleteAccountSection } from '@/features/settings/DeleteAccountSection'
 
@@ -17,6 +18,7 @@ function SettingsPage() {
     <div className="mx-auto flex w-full max-w-2xl flex-col gap-6 px-6 py-10">
       <h1 className="text-2xl font-semibold tracking-tight">{t('nav.settings')}</h1>
       <ChangePasswordForm />
+      <ChangeUserNameForm />
       <TelegramLinkCard />
       <DeleteAccountSection />
     </div>