feat(web): drop the empty terms checkbox from registration

The register step asked the user to accept "the terms of use" with no terms
linked anywhere — ceremony accepting nothing on a friends-only tool (UX review
C4). Remove the checkbox and the server-side acceptance requirement; only a
display name is required now.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/registration.go

@@ -115,10 +115,9 @@ func (a *App) handleRegister(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	displayName := strings.TrimSpace(r.FormValue("display_name"))
-	accepted := r.FormValue("accept_terms") != ""
-	if displayName == "" || !accepted {
+	if displayName == "" {
 		a.renderStatus(w, r, http.StatusBadRequest,
-			RegisterPage(user.Email, "Enter a display name and accept the terms to continue."))
+			RegisterPage(user.Email, "Enter a display name to continue."))
 		return
 	}