fix: blank expanded dimension table while data loads — show a spinner in DimensionTable

[nishantmonu51]

• Since Fix DimensionTable virtualized jankiness #8320 added contain: content to the virtualizer grid in DimensionTable.svelte, the Loading... and No results to show states have been clipped and invisible: with no rows, the grid is only header-height, and the text sat below it at mt-[30vh]. Expanding a dimension on a large dataset therefore showed a completely blank table until the query returned.
• Moved the empty/loading state out of the clipped virtualizer div into a centered overlay on the table container.
• While fetching, show a DelayedSpinner (same pattern as leaderboard headers; 300ms delay so fast loads don't flicker). The No results to show empty state is visible again as well.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[nishantmonu51]

@codex: review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: b8202c194c

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