Update dashboard shell with Tailwind

[BojayL]

Summary

• Refactor the dashboard shell and shared navbar components from CSS modules to Tailwind utility classes.
• Preserve desktop navigation, mobile menu behavior, active link styling, configuration link, and logout action.
• Keep the dashboard profile reminder responsive and remove the now-unused dashboard/navbar CSS modules.

Closes #51

Validation

• npm --workspace frontend run build
• yarn workspace frontend build
• Responsive browser check for /dashboard/my-courses covering desktop navigation and mobile menu behavior
• git diff --check

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