- Change cookie auth SameSite from Strict to Lax so Discord OAuth callback
can see existing Telegram auth session and perform linking instead of
creating a new standalone Discord session (root cause of broken linking).
- Add linking logic to /auth/telegram endpoint for Discord→Telegram linking.
- Add Telegram Login Widget in Profile.razor for Discord users.
- Add CookieAuthOptionsTests to verify Lax SameSite configuration.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
