fix: restore button visibility and unzip path validation for backups

[aayushbaluni]

Problem

The backup restore button disappears after certain interactions, and unzip operations can fail silently when given directory paths instead of file paths.

Change

• Fix the restore button disabled/visibility state logic
• Add path validation for backup restoration to reject directories
• Surface unzip errors properly instead of swallowing them

Fixes #4331.

Made with Cursor

[finnvyrn]

Sorry, I made a mistake. Please see #4331 (comment). If you can see the Restore button after horizontally scrolling the bar, maybe this pull request is no longer needed.

[aayushbaluni]

Thanks for the follow-up @finnvyrn! I see that the Restore button is indeed accessible via horizontal scroll. However, this PR still improves UX by ensuring the button remains visible without requiring horizontal scrolling when filenames are long — which aligns with your suggestion to "always show the Restore button." Happy to close if the maintainers feel it's not needed though.