fix: Navigate group blocks (#68)

* fix: Apply select default WordPress form styles

GutenbergKit does not load these WP Admin styles, so we must manually
apply them to ensure consistency and avoid elements overflowing their
parent containers.

* fix: Simplify layout styles with box-sizing border-box

This also matches Gutenberg's `edit-post` package approach.

* docs: Document style overrides

Mitigate confusion regarding which UI styles are overridden.

* refactor: Remove inspector icon adjustments

These subtle adjustments are not worth the maintenance burden of
manually overriding core styles.

* refactor: Consolidate duplicative control label styles

Separating the style declarations reduces clarity.

* refactor: Remove duplicative card title style declarations

These same styles are applied with other selectors.

* refactor: Remove seemingly unused styles

* refactor: Remove seemingly unused styles

* refactor: Remove duplicative panel body title style declarations

These same styles are applied with other selectors.

* docs: Document rationale for hiding the inserter between blocks

This inserter is displayed on hover, which is unavailable on touch
devices. Also, its presence steals focus, requiring two taps to insert a
block from the block inserter. Once to trigger hover and focus, once to
activate the button.

* fix: Navigate group blocks

Previously, the block appender--i.e., the inline block inseter--was
forcibly hidden. This caused group blocks--Group, Columns, Row, etc--to
collapse visually, leaving the near-impossible to select or modify.
Reintroducing the block appender allows traversing and modifying group
blocks.

* fix: Remove the block toolbar border radius

The border radius looks odd for the block toolbar that sits flush
against the screen edge.

* fix: Hide inline block appender button

This button's positioning and size is no ideal for small screens.

* task: Capture build output

* fix: Snackbar position and visibility

A recent refactor changed the CSS selector name, causing the styles to
no longer be applied.

* task: Capture build output

Author: dcalhoun