feat: Improve migration UX on login page

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- Show 'Database Migration Required' heading instead of welcome message when migrations pending
- Add warning alert explaining credentials needed to authorize migration
- Change button text from 'Go' to 'Migrate' during migration flow
- Add language strings for clear migration messaging

This makes it clear to the user that they are performing a migration
authorization step (not a regular login), and will need to re-authenticate
after migrations complete.

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