fix: hardcoded chart colors in PrototypeLibraryPortfolio break dark mode

[NhanLuongBGSV]

Context

In PR #413, chart datalabel colors in PrototypeLibraryPortfolio.tsx were changed from CSS variables to hardcoded hex values to work around Chart.js datalabels plugin not resolving hsl(var(...)) strings.

Problem

Hardcoded colors (#f3f4f6, #d1d5db, #374151) won't adapt to dark mode or custom themes.

File: frontend/src/components/organisms/PrototypeLibraryPortfolio.tsx (lines 220-230)

Current workaround

The hovered state already uses getCSSVariable('--primary') which works. Only the non-hovered states are hardcoded.

Suggested fix

Use getCSSVariable() for all color properties, same pattern as the hover state:

backgroundColor: (context) =>
  context.dataIndex === hoveredIndex
    ? `hsl(${getCSSVariable('--primary')})`
    : `hsl(${getCSSVariable('--muted')})`,
borderColor: `hsl(${getCSSVariable('--border')})`,
color: (context) =>
  context.dataIndex === hoveredIndex
    ? '#ffffff'
    : `hsl(${getCSSVariable('--muted-foreground')})`,