Refactors the admin dashboard bar charts to use CSS custom properties (`--bar-height`) instead of fragile inline `height` styles.
Changes include:
- Updating the HTML templates to pass the height as a CSS variable.
- Converting the `.bar-chart` layout from Flexbox to CSS Grid for more consistent column distribution.
- Using absolute positioning for `.bar-chart-bar` inside `.bar-chart-track`.
- Adding a Go test to verify that the dashboard renders the CSS variable and no longer uses inline height styles.