[style] ensure all floating menus have same gap from screen edges (#6120)

## Summary

Ensures all the floating shell app elements (graph canvas menu, minmap,
floating sidebar) all have .5rem gap between edge and viewport. Was not
proper distance for graph canvas menu and minimap due to splitter
overlay (parent) itself having 8px gap from viewport.

**After**:

<img width="605" height="535" alt="Screenshot from 2025-10-18 01-34-51"
src="https://github.com/user-attachments/assets/215efd5c-541f-441a-8b0d-1eb1f6fce672"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6120-style-ensure-all-floating-menus-have-same-gap-from-screen-edges-2906d73d365081598ae4d16f2380baf8)
by [Unito](https://www.unito.io)

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Co-authored-by: github-actions <[email protected]>

Author: christian-byrne