Make drag triggers consistent across the app

[SiobhyB]

Related to 6805-gh-Automattic/dotcom-forge, #28, and #29

Proposed Changes

• The bordering area of the onboarding and main screens have been made draggable. This is in response to feedback around expecting the top area of the app to always be draggable, as well as expectations for more areas of the app to be draggable than the slim border.

Onboarding	Main screen

• In addition, the left sidebar is now draggable (excluding the interactive buttons).

Testing Instructions

Onboarding screen

• Delete any sites from the app to view the onboarding screen.
• Verify it is now possible to drag the top area of the screen's white half, as per the guidance at Dragging doesn't work for the right (white) part of the initial app window #28 (comment).
• Verify that all components in the "Add your first site" form can still be interacted with.

Main screen

• When local sites have been added to the app, verify that the main area of the left sidebar is draggable.
• Also, verify that the bordering areas of the screen's white section can be dragged, in keeping with the onboarding screen.
• Verify that all components can still be interacted with as expected e.g. it should still be possible to click the buttons in the left sidebar.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[derekblank]

I noted on the Onboarding screen that there is some non-draggable whitespace above and below the Add Site form. It seems that if the app-no-drag-region class were moved one more element further down, the areas in purple would be draggable as well, still leaving the form itself interactable. I wasn't sure if leaving this space non-draggable was intentional, however, so disregard if so.

<div className="flex flex-col items-start self-stretch gap-6  app-no-drag-region">

Example

draggable.mov

[SiobhyB]

That's a good thought, I've gone ahead to implement your suggestion in f8a790d.

I had been a bit conservative with the amount of "draggable space" I was introducing, but agree that for the onboarding screen, it would feel a bit odd for all of that white space to not be draggable while the blue sidebar is completely draggable.

[derekblank]

I observed the draggable areas mentioned in the PR description have been improved. 👍 Just left a comment for potentially increasing the area even further on the Onboarding screen, but it's not at all a blocker. Otherwise, LGTM! 🚀