feat(register): redesign register page to match spec

Replaces the minimal login-style form with the full spec design:
hero section (eyebrow, headline, subtext), three labelled form sections,
2-column name grid, confirm-password field with client-side match hints,
password strength indicator, notification checkbox card, loading state on
submit, and "already have an account?" footer link.

Backend: adds notifyOnMention to RegisterRequest and wires both
notifyOnMention and notifyOnReply via updateNotificationPreferences on
invite redemption.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/dto/RegisterRequest.java

@@ -15,4 +15,5 @@ public class RegisterRequest {
     private String password;
     private String firstName;
     private String lastName;
+    private boolean notifyOnMention = true;
 }

backend/src/main/java/org/raddatz/familienarchiv/service/InviteService.java

@@ -91,6 +91,8 @@ public class InviteService {
                 token.getGroupIds()
         );
 
+        userService.updateNotificationPreferences(user.getId(), dto.isNotifyOnMention(), dto.isNotifyOnMention());
+
         token.setUseCount(token.getUseCount() + 1);
         inviteTokenRepository.save(token);