Existing-user invite flow shows registration form and fails with raw DB error on submit

[iamtanuj18]

When a user who already has a CTRL account is invited to a new study, the current flow is rough:

• Not logged in, clicking the invite link opens the full registration form. Filling everything in and submitting fails with Error Registering: emailHash already in use. That raw DB-ish message comes from application/backend/src/middlewares/ErrorHandler.ts using the Prisma unique constraint column name as-is, and shouldn't be exposed to end users. Having users fill the entire form only to fail on submit is bad UX too.
• Logged in as the invited user, clicking the same invite link still shows the registration form. Would expect them to just see and accept the invite from their dashboard (that flow already exists via GET /invites/pending + StudyInvites.tsx).
• The registration form doesn't pre-fill the email from the invite. Users could type a slightly different email and only find out on submit that it doesn't match the invite.

Suggested fix

The accept-invite flow for existing users already works in the dashboard (GET /invites/pending + POST /invites/{inviteId}/accept + StudyInvites.tsx). Simplest fix would be at the email-sending step in sendInvite (application/backend/src/controllers/ParticipantsController.ts):

• If a user with the recipient's email already exists: send a "you've been invited to , please log in to accept in your dashboard" email with a login link
• If not: send the current registration email with the /register/{inviteId} link