fix(ui): add skeleton loading states to Room Contextual Bar views

[sharanyamahajan]

Proposed changes

This PR adds skeleton loading placeholders to the Room Members view in the
Room Contextual Bar while data is being fetched.

Previously, the list area briefly rendered a blank state (or caused layout
shifts) during loading, especially when searching or switching filters.
The new skeletons preserve the list structure (avatar + text) during loading,
improving perceived performance and UX.

This is a UI-only change and does not modify any data fetching or business
logic.

Screenshots

• Before: List disappears briefly while loading
• After: Skeleton placeholders maintain layout until data is ready

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Issue(s)

Closes RocketChat#38162

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Steps to test or reproduce

1. Open any room
2. Open the Room Contextual Bar → Members
3. Use the search input or change filters
4. Observe the loading behavior

Expected result

• Skeleton placeholders are shown while data is loading
• No blank state or layout shift occurs
• List renders normally once data is loaded

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Further comments

• Scope is intentionally limited to the Room Members contextual bar
• Change is visual-only and aligned with existing skeleton patterns used
  elsewhere in the app (e.g., Sidebar)
• Follow-up improvements (Files / Mentions tabs) can be addressed separately
  if desired