fronted(modal): fix the position of modal overlay

[ChayanDass]

Fix: Properly Center Modal Overlays on Screen

#2472

Description

This PR fixes the modal overlay positioning issue where modals were not properly centered on the screen.

Problem

Modal overlays were rendering at incorrect positions (sometimes at the bottom, sometimes at random positions) instead of being centered both horizontally and vertically on the screen.

Affected Modals

• Add Service modal (Settings → Pinning Services)
• Generate Key modal (Settings → IPNS)
• CLI Tutor Mode modal (Files page)

Changes Made

1. Modal Positioning - Fixed modal rendering position using flexbox centering
2. Overlay Component - Updated to use proper centering utilities
3. Documentation - Added comprehensive issue documentation

Testing

✅ Add pinning service modal - properly centered
✅ IPNS key generation modal - properly centered
✅ CLI tutor mode modal - properly centered

All modals now appear centered both horizontally and vertically on the screen.