fix(UI): render Backup controls as <form> elements

[AprilSylph]

Description

• Relates to feat(UI): move sponsor progress to Configuration tab #2153 (comment)

The primary motivation for this was to show both subsections at the same time, making the contents of the Backup tab leave less empty space on mobile and other fixed-height situations.

Once I started pulling threads, though, I saw how much of an accessibility mess these controls are, so I ended up halfway to reimplementing them.

Before	Before	After

Testing steps

Smoke-test the Backup tab. Every element within it should work pretty much as before.

Expected changes include:

• Both "Export" and "Import" sections are visible at the same time
• The "Export" textarea no longer shrinks if the addon storage has less than eight keys
• The "Export" textarea no longer selects all of its text content on click
• Spacing is slightly different (should be more reminiscient of the Links tab now)