Improved responsive panel behaviour, sidebar popovers, and support for private assets

This update makes responsive layout state more reliable across viewport changes, improves popover positioning in the sidebar, prepares for Sanity Studio to support private asset selection, and adds a new warning when the Media Library is using cookie authentication and some features may not be supported.

Studio’s configured with Media Library can now report that they support private assets. This unlocks the ability to select private assets from the Media Library and access those assets in applications using signed URLs.

For older Studios that don't report this new capability, Media Library keeps the existing behavior and continues to force public only assets to preserve backwards compatibility.

The layout and panel state handling is now more consistent across breakpoints. Updates are driven by media queries rather than focus-based behavior, making resizing more reliable. Navigation and edit panels now track narrow vs wide viewports more accurately, avoiding issues like mobile menus reopening on reload or edit panels appearing blank, and behaving more predictably when switching viewport sizes.

A new warning banner is shown when Media Library is running without a token, helping explain why certain functionality may not be available. This can happen in scenarios where a studio is running outside of Dashboard.

• Aspect popovers no longer get clipped by the sidebar footer. Popovers in the sidebar now use an explicit boundary tied to the sidebar scroll area, improving positioning and avoiding cutoffs.
• Fixes aspect hover UI overlapping sticky headers. Aspect UI elements no longer render above sticky navigation/header elements on hover.