refactor(ui): live preview iframe loader (#16004)

Continuation of https://github.com/payloadcms/payload/pull/15999.

Without optimistically loading the live preview window, the initial
loading state is left unhandled when toggling into live preview for the
first time. This causes users to see blank content until the iframe has
loaded, without indication that something is happening.

Now, we display a loading indicator while the iframe request is in
flight, that notably:
1. Only shows if the load takes longer than x ms.
2. If shown, is forced to remain in place for _at least_ x ms to avoid
flickering, even if the request finishes before then.

Before:


https://github.com/user-attachments/assets/70852e80-ac0e-4b96-b59e-8817c65629a6

After:


https://github.com/user-attachments/assets/5fda30d9-8c87-42bc-ba20-a8f7ae8f1245

Related:
- The `ShimmerEffect` component now accepts arbitrary HTML attributes
and supports a new `transparent` prop

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