Adds a "Connect Discord" button in the Account settings section that
triggers BetterAuth's linkSocial flow to attach a Discord account to
an existing user without requiring a sign-out/sign-in. After the OAuth
callback, the ?section= URL param returns the user to the Account tab.
The Notifications toggle was already gated on Discord being linked.
https://claude.ai/code/session_01UerTpTuBWjreTNEns8srkp
