feat: add Image-Charts backend (hosted-image-URL render target) - #95

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

'yMinGroupStep' is assigned a value but never used. Allowed unused vars must match /^_/u

'xMinGroupStep' is assigned a value but never used. Allowed unused vars must match /^_/u

'maintainContinuousAxisRatio' is assigned a value but never used. Allowed unused vars must match /^_/u

'minSubplotVal' is assigned a value but never used. Allowed unused vars must match /^_/u

'AxisLayoutInput' is defined but never used. Allowed unused vars must match /^_/u

'chartType' is defined but never used. Allowed unused args must match /^_/u

'groupNames' is assigned a value but never used. Allowed unused vars must match /^_/u

'chartProperties' is assigned a value but never used. Allowed unused vars must match /^_/u

'isRadar' is assigned a value but never used. Allowed unused vars must match /^_/u