fix: canvas dashboard flashes 404 on load due to race condition

[djbarnwal]

On page load, CanvasInitialization ran two independent queries: the authoritative createQueryServiceResolveCanvas (which returns validSpec) and a secondary useResource call (which returns lightweight resource metadata). The reconcileErrorMessage derived state used both as sources:

$: reconcileErrorMessage = !validSpec
  ? reconcileError || resource?.meta?.reconcileError
  : undefined;

The useResource query could resolve first — before fetchedCanvas loaded — with a reconcileError already set (e.g. "dependency error: ..."). At that moment validSpec was still undefined, so CanvasLoadingState would briefly show the 404 page before the resolve-canvas response arrived.

Changes

• Removed the resource?.meta?.reconcileError fallback from reconcileErrorMessage.
  Error state is now driven solely by the resolve-canvas query, which is the authoritative source for canvas readiness.

https://linear.app/rilldata/issue/APP-721/canvas-dashboard-flashes-with-404-error-before-loading

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!