Preview form definition

## User stories

As a PM who has uploaded a new form
I want to preview my form
So that I can confirm it works the way it was intended

As a PM who has a form with location questions
I want to see the appearances what the data collector will see (rather than seeing all map functionality, as with editing)
So that I can confirm the form works as intended

Given I want to preview a draft
When I click preview, and I'm taken to a new tab,
Then there should be a clear visual indicator that this is a "preview"
So that I can safely test things and not make any mistakes

Given I'm previewing my form
When I get to the end of the form
Then I can validate it works

Given I'm previewing a draft of my form
When I toggle between the mobile/desktop icons
Then I can see what the form looks like for end users before publishing

The issue does not name any files, tests, or entry points. Start by locating the form editor and rendering flow, then trace how a draft is opened and how mobile/desktop views are selected. Done means a draft can open in a clearly marked preview, location questions show collector-facing behavior, the form can be completed, and viewport toggling works.

Written by the indexing model from the issue text.