web: add download snapshot dialog behind feature flag

[lizzthabet]

• Add a new snapshot dialog for downloading a snapshot file directly, only visible behind a feature flag; this dialog is styled like the other menu item dialogs.
• Add a new feature flag named offline_snapshot_creation
• Add a dialog anchor to the HUD state for the float dialog to display. (Because snapshot creation grabs data from the HUD state, I opted for this change, rather than a larger refactor of the SnapshotProvider state to move data there so that the float dialog could live in the GlobalNav.)

Thanks to @landism for doing the majority of this PR! Leaving this in draft mode until I have a couple tests added, but otherwise nothing else should change.

Closes ch13474.

[shortcut-integration]

This pull request has been linked to Shortcut Story #13474: As user, download snapshot.json via Tilt Web.

[landism]

I haven't figured out why, but this breaks online snapshot creation for me:

(and apparently this was true in my original branch as well).

I'll update if I figure it out before EOD.

[landism]

I'll update if I figure it out before EOD.

pushed a fix!