Implement dark mode functionality in the insights-chrome application (chrome-ui shell for console.redhat.com).

Implementation Details:

• Store active mode in useChrome hook (chrome context)
• Follow PatternFly dark mode guidelines: https://www.patternfly.org/developer-resources/dark-theme-handbook/#enabling-dark-theme
• Provide toggle mechanism for users to switch between light/dark modes
• Persist user preference
• Read initial setting from browser data

Codebase: insights-chrome
Parent Epic: RHCLOUD-39367 (https://issues.redhat.com/browse/RHCLOUD-39367)