fix(frontend): prevent overscroll artifacts without changing global page background

[PRAteek-singHWY]

Summary

This PR fixes a global UI regression where multiple pages appeared with an unintended dark/blue background after the overscroll patch.

Root cause

A recent overscroll fix changed body background globally in application/frontend/src/app.scss.
That solved one visual gap, but it also changed baseline page appearance across the site.

What this PR changes

• Restores normal global page background behavior.
• Scopes overscroll handling to app viewport behavior instead of forcing global dark body color.
• Uses #mount as the scroll container and disables vertical overscroll bounce artifacts there.

Files changed

• application/frontend/src/app.scss

Result

• No unintended global dark/blue tint on pages like /root_cres, /map_analysis, /chatbot, etc.
• Overscroll white/flash artifact is removed.

Validation

• Verified behavior on affected routes from screenshots/video.
• Confirmed page background remains correct while overscroll artifact no longer appears.

Media

• Before

Screen.Recording.2026-02-28.at.04.01.08.1.mov