fix(admin): use theme-aware grid color for health gauge background track

[De-pitcher]

Description

Fixes #856 by replacing hardcoded low-opacity white gauge tracks in OverviewTab with the theme-aware palette grid color from chartTheme(useEffectiveTheme()).grid.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Test addition or update

Related Issue

Closes #856

Changes Made

• Imported chartTheme from @/lib/charts/palette in src/components/admin/tabs/OverviewTab.tsx.
• Updated HeroStatusBanner and MetricGauge to resolve the current effective theme via useEffectiveTheme() and use chartTheme(theme).grid for the RadialBar background track fill.
• Updated tests/helpers/mock-monaco.ts to surface background.fill on mock RadialBar elements via data-bg.
• Added unit tests in tests/components/admin/OverviewTab.test.tsx verifying that health and metric gauge tracks use #222222 in dark mode and #e4e4e7 in light mode.

Testing

• I have tested this locally
• I have added/updated tests
• All existing tests pass

Test Environment

• OS: Windows 11
• Node.js/Bun Version: Bun 1.3.11

Checklist

• My code follows the project's code style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[De-pitcher]

Closing in favor of PR #946 for issue #862 per maintainer recommendation.