feat: implement WCAG-compliant high-contrast theme (#778)

[Marvelg256]

• Add CSS custom property overrides for [data-high-contrast="true"] in both dark and light themes, with a fallback for missing data-theme
• Enhanced forced-colors media query for system-level high-contrast mode
• Add CHART_COLORS_HIGH_CONTRAST and CHART_COLORS_HIGH_CONTRAST_LIGHT frozen palettes for chart components with >= 7:1 contrast ratios
• Add STATUS_COLORS / STATUS_COLORS_HIGH_CONTRAST and getStatusColors() helper for semantic status indicators
• Add getChartColors(isHighContrast, theme) resolver for automatic palette selection based on current theme + accessibility state
• Write 32 comprehensive tests: primary flow (8), WCAG compliance (9), boundary cases (6), failure cases (7), state transitions (2)
• Add developer documentation in docs/HIGH_CONTRAST_THEME.md with architecture, usage, contrast ratios, and migration guidance

Closes #778

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