feat(ui): replace --bui-bg-popover with layered bg approach

Remove the --bui-bg-popover token and adopt a two-layer background
pattern across overlay components: outer shell uses --bui-bg-app,
inner content uses Box bg="neutral-1".

Components updated: Popover, Tooltip, Menu, Dialog.

Each component gets a local border-radius CSS variable, a Box
content wrapper with neutral-1 background, and updated arrow
fills where applicable.

Story "is open" variants for each component now use a dot-grid
background pattern to detect background transparency issues in
Chromatic snapshots.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

docs/conf/user-interface/index.md

@@ -156,7 +156,6 @@ These colors form a layered neutral scale for your application backgrounds. `--b
 | Token Name                    | Description                                                  |
 | ----------------------------- | ------------------------------------------------------------ |
 | `--bui-bg-app`                | The base background color of your Backstage instance.        |
-| `--bui-bg-popover`            | The background color used for popovers, tooltips, and menus. |
 | `--bui-bg-neutral-1`          | First elevated layer. Use for cards, dialogs, and panels.    |
 | `--bui-bg-neutral-1-hover`    | Hover state for elements on neutral-1.                       |
 | `--bui-bg-neutral-1-pressed`  | Pressed state for elements on neutral-1.                     |