Components: Add hairline border to Modal frame

Match the always-on token-colored hairline border now present on the
Dialog/AlertDialog/Drawer/Popover overlays, so the popup edge reads
as a defined boundary against the lighter tokenized backdrop instead
of relying on the elevation shadow alone.

`@include reset` already applies `box-sizing: border-box`, so the 1px
border is absorbed into the existing frame dimensions and does not
shift layout.

Made-with: Cursor

Author: jameskoster

packages/components/CHANGELOG.md

@@ -7,7 +7,7 @@
 -   `Menu`: Remove `cursor: not-allowed` and added pointer styles to menu ([#70412](https://github.com/WordPress/gutenberg/pull/70412))
 -   `FormToggle`: Add stacking context isolation ([#77619](https://github.com/WordPress/gutenberg/pull/77619)).
 -   `FormTokenField`: Add `help` prop to render additional help text below the field, and deprecate the `__experimentalShowHowTo` prop in favor of it. The `help` prop now defaults to the previous how-to text; pass an empty string to hide it ([#77552](https://github.com/WordPress/gutenberg/pull/77552)).
--   `Modal`: Render the backdrop with the `--wpds-color-bg-surface-neutral-weak` system token instead of a hardcoded black, and bump the alpha to 75% so the lighter token retains comparable presence — matching the Dialog/Drawer/Popover backdrop styling.
+-   `Modal`: Add a token-colored hairline border to the modal frame, and render the backdrop with the `--wpds-color-bg-surface-neutral-weak` system token instead of a hardcoded black at 75% alpha — matching the Dialog/Drawer/Popover overlay styling.
 
 ### Deprecations
 

packages/components/src/modal/style.scss

@@ -36,6 +36,7 @@
 	margin: $grid-unit-50 0 0 0;
 	width: 100%;
 	background: $white;
+	border: var(--wpds-border-width-xs) solid var(--wpds-color-stroke-surface-neutral);
 	box-shadow: $elevation-large;
 	border-radius: $radius-large $radius-large 0 0;
 	overflow: hidden;