Block Editor: Fix multi-select inspector padding

[aduth]

Fixes #17975.
Possibly related: #17880 (cc @jorgefilipecosta ?)

This pull request seeks to resolve an issue where the block inspector does not have sufficient padding for multi-selections. It appears that the sidebar area previously had some default padding that was intended to be offset by the multi-inspector, and that this padding no longer exists. The inspector can simply apply its own padding.

These changes also use the variable $grid-size-large to align with that of the single block inspector card (see #17880). There should be no effective change, since $grid-size-large is the same 16px value.

Before	After

Testing Instructions:

1. Navigate to Posts > Add New
2. Insert multiple blocks
3. Select multiple blocks
4. Verify padding displays as expected in sidebar inspector title

[ZebulanStanphill]

Fixes #17975.