feat: Add comprehensive light/dark mode support with semantic CSS variables

- Implement CSS light-dark() function for automatic theme switching
- Refactor all color variables to use semantic naming (no light/dark in names)
- Add 26 light-dark() declarations for complete theme coverage
- Consolidate duplicate color values into reusable variables
- Replace all hardcoded colors with semantic variables
- Remove deprecated variables (--font-color-light, --bg-color-dark, etc.)
- Add theme-specific variables for tables, toasts, shadows, and UI elements
- Ensure consistent theming across all interface components

Author: baelter