fix: Improve sidebar behavior and layout responsiveness

[Ayushdas1904]

closes #857
Description:

1. Implemented active-tab highlighting using useLocation/isActive.
2. Added logout handling (handleAuth + redirect to /login).
3. Adjusted CSS for collapsed sidebar — icons now stay centered.
4. Introduced --sidemenu-width variable and synced it via useEffect on collapse/expand.
5. Updated Main.jsx, Dashboard.jsx, and LearningRoadmap.jsx to shift with sidebar width.
6. Centered LearningRoadmap content with a responsive, max-width layout.
7. Added smooth transitions for width/margin changes.

Screen.Recording.2025-10-31.at.09.45.34.mov

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[avinash201199]

Please resolve the conflicts

[Ayushdas1904]

@avinash201199