Add skeleton loader for file tree browser

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Context

See #582189 (comment 2927656198).

Do we want to have skeleton loader?

Environment: Production Browser: Chrome 142 OS: macOS Viewport: Desktop Screen Reader: [If applicable: VoiceOver, NVDA, JAWS, Orca - include version]

Evidence

it's a small delay with my internet. can definitely get worse

Proposal

Use a skeleton loader to load chunks of the file tree browser when there is a large list.