UI: Import: Continue button

Context

Once the user clicks 'Import your files' from the dashboard, they are taken to an import page. On this page we need a 'Continue' button so the user can continue to the Book title step.

We can show the continue button whether or not the user added files, because they can always add more files in the editor step.

Design

Add the 'Continue' button to the Import page as in the Figma design here and here and shown in the two mockups below:

Implementation (if applicable)

[A description of the steps to implement the feature.]

Alternative approaches (if applicable)

[Include any alternatives to meet this use case.]

Open issues (if applicable)

[Links to and a discussion of related issues, if applicable.]