fix: resolve #472 — add z-index to Drawer root element

The Drawer component's root element was missing a z-index, causing it
to appear behind elements with even a z-index of 1. This fix adds
z-index support to the Drawer root container via the --gn-drawer-z-index
CSS variable.

Changes:
- Added z-index: var(--gn-drawer-z-index) to Drawer root element
- Updated README to document the new --gn-drawer-z-index CSS variable
- No default value provided to avoid breaking existing layouts
- Users can set the CSS variable to control stacking order (e.g., 1000)

Author: DarkGenius

src/components/Drawer/Drawer.scss

@@ -4,6 +4,7 @@ $block: '.#{variables.$ns}drawer';
 
 #{$block} {
     pointer-events: none;
+    z-index: var(--gn-drawer-z-index);
 
     --_--item-shadow-default: 0 1px 5px 0 var(--g-color-sfx-shadow);
     --_--item-position: fixed;

src/components/Drawer/README.md

@@ -99,6 +99,8 @@ The Drawer module consists of two primary components: `Drawer` and `DrawerItem`.
 
 | Name                                          | Description                                                 |                 Default                 |
 | :-------------------------------------------- | :---------------------------------------------------------- | :-------------------------------------: |
+| Drawer                                        |                                                             |                                         |
+| `--gn-drawer-z-index`                         | z-index of the drawer root container                        |                `"auto"`                 |
 | DrawerItem                                    |                                                             |                                         |
 | `--gn-drawer-item-position`                   | The position of the drawer item on page                     |                 `fixed`                 |
 | `--gn-drawer-item-shadow`                     | The box-shadow of the drawer item when veil is hidden       | `0 1px 5px 0 var(--g-color-sfx-shadow)` |