feat: support CSS values for container_height option (#587)

[GovardhaneNitin]

Summary

This PR implements smart type detection for the container_height option, allowing CSS string values like percentages, calc(), viewport units, and other CSS values in addition to numeric pixel values.

Fixes #587

Problem

The container_height option previously only accepted numeric pixel values or 'auto'. When users passed CSS string values, they were incorrectly converted:

container_height: '100%'              // Resulted in: --gv-grid-height: 100%px (invalid CSS)
container_height: 'calc(100vh - 200px)' // Resulted in: calc(100vh - 200px)px (invalid CSS)

This made it difficult to create responsive, viewport-constrained Gantt charts with proper scrolling behavior.

Solution

The fix uses smart type detection:

• String values → used as-is (e.g., '100%' → '100%')
• Numeric values → append 'px' as before (e.g., 600 → '600px')

Usage

new Gantt(container, tasks, {
    container_height: '100%',                    // ✅ Sets: --gv-grid-height: 100%
    container_height: 'calc(100vh - 200px)',     // ✅ Sets: --gv-grid-height: calc(100vh - 200px)
    container_height: '50vh',                    // ✅ Sets: --gv-grid-height: 50vh
    container_height: 600,                       // ✅ Still works: --gv-grid-height: 600px
});

Backward Compatibility

• ✅ Numeric values still work exactly as before
• ✅ 'auto' value behavior unchanged
• ✅ No breaking changes