Bundle bpmn-js assets locally instead of loading from CDN

[Copilot]

Addresses feedback from #60 to eliminate runtime internet dependency for diagram rendering by bundling bpmn-js assets with the CLI.

Changes

• Dependency: Added bpmn-js@18 as production dependency
• Build: Extended build script to copy bpmn-js assets (bpmn-viewer.production.min.js, CSS, fonts) from node_modules to dist/assets/bpmn-js/
• Runtime: Modified diagram.ts to use file:// URLs pointing to bundled assets instead of unpkg.com CDN

Implementation

Before:

<link rel="stylesheet" href="https://unpkg.com/bpmn-js@18/dist/assets/bpmn-js.css">
<script src="https://unpkg.com/bpmn-js@18/dist/bpmn-viewer.production.min.js"></script>

After:

const assetsDir = join(__dirname, '..', 'assets', 'bpmn-js');
const cssUrl = `file://${join(assetsDir, 'assets', 'bpmn-js.css')}`;
const jsUrl = `file://${join(assetsDir, 'bpmn-viewer.production.min.js')}`;

Diagram rendering now works offline and in air-gapped/proxied environments.

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