fix: show aggregation errors in Chart.svelte instead of an infinite spinner

[eminemead]

• Failed aggregations left the chart on an infinite diamond spinner.
• Chart.svelte checked isFetching before the query error, so a failed request never left the loading state.
• Render the aggregation error first; keep the spinner only when there is no error.

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!

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Developed in collaboration with Claude Code