feat: apply visible lazy-loading guard to canvas chart components

[djbarnwal]

Canvas components use an IntersectionObserver to set a visible store to true the first time a component scrolls into the viewport. This pattern was already applied to KPI and leaderboard components to prevent data queries from firing off-screen, but chart components were not covered.

Threads visible: Readable<boolean> from CanvasChart.svelte → getChartDataForCanvas → BaseChart.createChartDataQuery → each chart provider

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!

[ericpgreen2]

Looks good 👍