Migrate Welcome route page to app router

[boodland]

Sub issue of #702

Move welcome page route to app router. Steps:

1. Create E2E test to cover the route
2. Create welcome app segment containing the dynamic partnerName segment with page file (server component) and partner-name file (client component).

[boodland]

@kyleecodes, @eleanorreem assign it to me please

[github-actions]

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[github-actions]

As per Chayn policy, after 30 days of inactivity, we will be unassigning this issue. Please comment to stay assigned.