Add QR code login option to web invitation flows when BTCPay App Plugin is enabled

## Overview
Currently, when an admin creates an invitation for a user, the user can either manually input the URL or scan a QR code. However, when users click on the invitation URL directly, they're presented with only web-based login options. That could lead them into setting up the account on the web and then not really sure where they came from or what to do.

Neither of these screens currently offers a QR code option for mobile app login/setup.

For users who use the BTCPay mobile app, we should provide a QR code option on these web pages. Since it's only relevant for the app onboarding, I wonder if BTCPay App plugin could overwrite the login page and place a QR there.

## Current behavior
When a user clicks on an invitation link, they see one of two screens:
1. A login screen (when the admin has pre-set the password)
2. A password creation screen (when the user needs to set their own password)

## Proposed solution
Add a QR code section to both invitation flow screens that allows users to complete the process using the BTCPay mobile app:

- For pre-set password scenario: Add a "Sign in with BTCPay Mobile App" section with QR code
- For user-set password scenario: Add a "Complete Setup in Mobile App" section with QR code

### Current screens:
**Pre-set password scenario:**

**User-set password scenario:**

### Mock for QR implementation:

Start by locating the two invitation flow screens described in the issue: the pre-set-password login page and the user-set-password setup page. Trace how each page is rendered and how the BTCPay App plugin can provide onboarding data, then verify that both screens offer the requested mobile-app QR flow without disrupting web login or setup.

Written by the indexing model from the issue text.