fix: use unvalidated spec as fallback for canvas

[djbarnwal]

Previously, if any canvas component had an invalid spec, the entire canvas dashboard became inaccessible in Rill Developer because ResolveCanvas returned early when canvas.state.validSpec was nil. This broke the editing workflow where a user intentionally changes a metrics_view (causing a transient error state) before selecting the correct fields.

• Fall back to the unvalidated spec in ResolveCanvas when validSpec is nil, so the canvas shell remains accessible during mid-edit error states
• Apply the same fallback for component renderer properties in BaseChart, CanvasPivot, and BaseCanvasComponent
• Fall back to spec.renderer when reading component type in canvas-entity.ts and BaseChart/CanvasPivot, fixing a blank inspector panel when clicking a broken component

The error/loading banner UX in CanvasInitialization.svelte is unchanged — it still uses validSpec to drive reconcile error messaging correctly. This matches the fallback pattern already used in ResolveTransitiveAccess (reconcilers/canvas.go:169-172).

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!