Export Wallet fixes

[JSKitty]

Abstract

This small PR has various fixes for our Export Wallet feature:

• The file type in the File Export button is now correct (.json instead of .csv).
• The file export icon now renders correctly; previously, it was invisible.
• The private key text now word-wraps symmetrically; nicer on the eyes, uses less height, and easier to copy/paste.

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Testing

To test this PR, it's suggested to attempt these user flows, or variations of these:

• Check that the 'file export' button icon is displayed.
• Check that the plaintext export data word-wraps symmetrically and evenly.
• Check that the file export's file type is the correct type (json).

If any errors are found, the PR works unexpectedly, or you have viable suggestions to improve the UX or functionality of the PR, let me know!

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