Allow deletion of unused assets

[Copilot]

The asset editor hides Delete for assets whose block-reference metadata still contains IDs from deleted blocks. This prevents users from removing unused named or unnamed assets.

• Reference cleanup
  • Refresh block asset metadata from the active Blockly workspace before evaluating asset usage.
  • Remove inactive block IDs from named assets without deleting the assets themselves.
  • Continue removing unnamed assets once no active block references remain.

if (blocksEditor?.editor) {
    project.removeInactiveBlockAssets(
        blocksEditor.editor.getAllBlocks(false).map(block => block.id)
    );
}

• Fixes CSEd Week 2020 blog post #7683