Fix: Memory leak in MainChart ColorSchemeChange listener

[codomposer]

Fix: Issue 452

Closes #444
Closes #355
Closes #329

Problem

The ColorSchemeChange event listener in MainChart was never cleaned up, causing memory leaks on component unmount.

Solution

Added cleanup function to remove the event listener when component unmounts.

Changes

• Extracted inline arrow function to named handleColorSchemeChange function
• Added return statement in useEffect to remove listener on unmount

Code

Before:

useEffect(() => {
  document.documentElement.addEventListener('ColorSchemeChange', () => {
    if (chartRef.current) {
      setTimeout(() => {
        chartRef.current.options.scales.x.grid.borderColor = getStyle(
          '--cui-border-color-translucent',
        )
        chartRef.current.options.scales.x.grid.color = getStyle('--cui-border-color-translucent')
        chartRef.current.options.scales.x.ticks.color = getStyle('--cui-body-color')
        chartRef.current.options.scales.y.grid.borderColor = getStyle(
          '--cui-border-color-translucent',
        )
        chartRef.current.options.scales.y.grid.color = getStyle('--cui-border-color-translucent')
        chartRef.current.options.scales.y.ticks.color = getStyle('--cui-body-color')
        chartRef.current.update()
      })
    }
  })
}, [chartRef])