feat: defer canvas pivot/table queries until component is visible

[djbarnwal]

Pivot/Table Visibility Gating for Canvas

This change extends the visibility-gating pattern (already used by KPI and chart components) to the canvas pivot/table component.

Key Changes

• Visibility-based query execution

  • Pivot queries now only fire after the component scrolls into view.
  • This uses the existing IntersectionObserver in CanvasComponent.svelte.

• Reactive enabled flag

  • PivotDashboardContext.enabled has been changed from:
    • boolean → Readable<boolean>
  • This allows the query’s enabled flag to react to visibility changes.

• usePivotForCanvas update

  • Now accepts:

    visible: Readable<boolean>

  • This value is passed through as the enabled context.

Explore Pivot Behaviour (Unchanged)

• The Explore pivot path (usePivotForExplore) remains unaffected.
• It wraps its existing static boolean in readable(...), preserving the always-enabled behaviour.

https://linear.app/rilldata/issue/APP-157/optimize-data-fetching-based-on-widget-visibility

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!