Add catch for disabled WebGL

[jdillard]

Cause: Sigma.js fails to initialize WebGL because the browser reports a null WebGLRenderingContext. The blendFunc call happens during WebGL setup.

Common reasons:

1. Browser or GPU WebGL disabled — older Safari versions, privacy modes, or hardware acceleration off.
2. Cross-origin iframe — WebGL blocked by CSP or sandbox flags.
3. Sigma fallback bug — if WebGL creation returns null, Sigma still assumes success instead of falling back to canvas.

Check:

const canvas = document.createElement('canvas');
const gl = canvas.getContext('webgl');
console.log(gl ? 'WebGL ok' : 'WebGL unavailable');

Fix options:

• Before creating the renderer, test for WebGL:

  if (!window.WebGLRenderingContext || !document.createElement('canvas').getContext('webgl')) {
    alert('WebGL not supported in this browser');
    return;
  }
  new sigma({ ... });

• Or force Sigma to use the canvas renderer if WebGL fails:

  try {
    renderer = new sigma.Renderer({ container, type: 'webgl' });
  } catch {
    renderer = new sigma.Renderer({ container, type: 'canvas' });
  }