fix: add explicit border-border to prevent color leaking from consumer CSS

Components using `astw:border` (border-width only) relied on the global
`* { border-color: var(--border) }` rule in `@layer base`. When consumers
import Tailwind CSS after app-shell styles, Tailwind's preflight resets
border-color to `var(--color-gray-200)`, causing thick gray borders.

Adding explicit `astw:border-border` at the utility layer ensures the
correct color regardless of CSS import order.

Author: IzumiSy

packages/core/__snapshots__/src__components__attachment__Attachment.test.tsx.snap

@@ -4,4 +4,4 @@ exports[`Attachment > snapshots > disabled 1`] = `"<div data-slot="attachment" c
 
 exports[`Attachment > snapshots > empty default 1`] = `"<div data-slot="attachment" class="astw:@container astw:flex astw:w-full astw:min-w-0 astw:flex-col"><input multiple="" class="astw:hidden" data-testid="attachment-upload-input" type="file"><div data-slot="attachment-content" class="astw:min-w-0"><div class="astw:flex astw:flex-wrap astw:gap-4"><button type="button" aria-label="Click to upload" data-testid="attachment-upload-tile" class="astw:flex astw:h-30 astw:w-[200px] astw:shrink-0 astw:cursor-pointer astw:flex-col astw:justify-between astw:rounded-lg astw:border-2 astw:border-dashed astw:border-border astw:bg-muted/50 astw:p-3 astw:transition-colors astw:hover:bg-muted"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-cloud-upload astw:size-5 astw:text-muted-foreground astw:opacity-60" aria-hidden="true"><path d="M12 13v8"></path><path d="M4 14.899A7 7 0 1 1 15.71 8h1.79a4.5 4.5 0 0 1 2.5 8.242"></path><path d="m8 17 4-4 4 4"></path></svg><div class="astw:min-w-0 astw:self-stretch astw:text-left"><p class="astw:text-sm astw:leading-normal astw:text-foreground">Click to upload</p></div></button></div></div></div>"`;
 
-exports[`Attachment > snapshots > populated mixed items 1`] = `"<div data-slot="attachment" class="astw:@container astw:flex astw:w-full astw:min-w-0 astw:flex-col"><input multiple="" class="astw:hidden" data-testid="attachment-upload-input" type="file"><div data-slot="attachment-content" class="astw:min-w-0"><div class="astw:flex astw:flex-wrap astw:gap-4"><div class="astw:group astw:relative"><div class="astw:relative astw:size-30 astw:shrink-0 astw:overflow-hidden astw:rounded-lg astw:bg-muted"><img alt="shoe-red.png" class="astw:size-full astw:object-cover" src="https://example.com/shoe-red.png"></div><div class="astw:absolute astw:top-2 astw:right-2 astw:opacity-0 astw:transition-opacity astw:group-hover:opacity-100 astw:group-focus-within:opacity-100"><button type="button" tabindex="0" aria-haspopup="menu" id="base-ui-_r_3_" data-slot="menu-trigger" aria-label="Attachment options for shoe-red.png" class="astw:inline-flex astw:size-7 astw:items-center astw:justify-center astw:rounded-md astw:border astw:bg-background/90 astw:text-foreground astw:shadow-xs astw:hover:bg-accent" aria-expanded="false"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-ellipsis astw:size-4" aria-hidden="true"><circle cx="12" cy="12" r="1"></circle><circle cx="19" cy="12" r="1"></circle><circle cx="5" cy="12" r="1"></circle></svg></button></div></div><div class="astw:group astw:relative"><div class="astw:relative astw:size-30 astw:shrink-0 astw:overflow-hidden astw:rounded-lg astw:border astw:border-border astw:flex astw:flex-col astw:justify-between astw:bg-card astw:p-3"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-file astw:size-5 astw:text-muted-foreground astw:opacity-60" aria-hidden="true" data-testid="attachment-file-icon"><path d="M6 22a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h8a2.4 2.4 0 0 1 1.704.706l3.588 3.588A2.4 2.4 0 0 1 20 8v12a2 2 0 0 1-2 2z"></path><path d="M14 2v5a1 1 0 0 0 1 1h5"></path></svg><p class="astw:flex astw:min-w-0 astw:items-end astw:gap-0.5 astw:text-xs astw:leading-normal astw:text-foreground"><span class="astw:min-w-0 astw:flex-1 astw:line-clamp-2 astw:break-all">Aug-Sep 2025_1234-12</span><span class="astw:shrink-0">.pdf</span></p></div><div class="astw:absolute astw:top-2 astw:right-2 astw:opacity-0 astw:transition-opacity astw:group-hover:opacity-100 astw:group-focus-within:opacity-100"><button type="button" tabindex="0" aria-haspopup="menu" id="base-ui-_r_8_" data-slot="menu-trigger" aria-label="Attachment options for Aug-Sep 2025_1234-12.pdf" class="astw:inline-flex astw:size-7 astw:items-center astw:justify-center astw:rounded-md astw:border astw:bg-background/90 astw:text-foreground astw:shadow-xs astw:hover:bg-accent" aria-expanded="false"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-ellipsis astw:size-4" aria-hidden="true"><circle cx="12" cy="12" r="1"></circle><circle cx="19" cy="12" r="1"></circle><circle cx="5" cy="12" r="1"></circle></svg></button></div></div><button type="button" aria-label="Click to upload" data-testid="attachment-upload-tile" class="astw:flex astw:h-30 astw:w-[200px] astw:shrink-0 astw:cursor-pointer astw:flex-col astw:justify-between astw:rounded-lg astw:border-2 astw:border-dashed astw:border-border astw:bg-muted/50 astw:p-3 astw:transition-colors astw:hover:bg-muted"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-cloud-upload astw:size-5 astw:text-muted-foreground astw:opacity-60" aria-hidden="true"><path d="M12 13v8"></path><path d="M4 14.899A7 7 0 1 1 15.71 8h1.79a4.5 4.5 0 0 1 2.5 8.242"></path><path d="m8 17 4-4 4 4"></path></svg><div class="astw:min-w-0 astw:self-stretch astw:text-left"><p class="astw:text-sm astw:leading-normal astw:text-foreground">Click to upload</p></div></button></div></div></div>"`;
+exports[`Attachment > snapshots > populated mixed items 1`] = `"<div data-slot="attachment" class="astw:@container astw:flex astw:w-full astw:min-w-0 astw:flex-col"><input multiple="" class="astw:hidden" data-testid="attachment-upload-input" type="file"><div data-slot="attachment-content" class="astw:min-w-0"><div class="astw:flex astw:flex-wrap astw:gap-4"><div class="astw:group astw:relative"><div class="astw:relative astw:size-30 astw:shrink-0 astw:overflow-hidden astw:rounded-lg astw:bg-muted"><img alt="shoe-red.png" class="astw:size-full astw:object-cover" src="https://example.com/shoe-red.png"></div><div class="astw:absolute astw:top-2 astw:right-2 astw:opacity-0 astw:transition-opacity astw:group-hover:opacity-100 astw:group-focus-within:opacity-100"><button type="button" tabindex="0" aria-haspopup="menu" id="base-ui-_r_3_" data-slot="menu-trigger" aria-label="Attachment options for shoe-red.png" class="astw:inline-flex astw:size-7 astw:items-center astw:justify-center astw:rounded-md astw:border astw:border-border astw:bg-background/90 astw:text-foreground astw:shadow-xs astw:hover:bg-accent" aria-expanded="false"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-ellipsis astw:size-4" aria-hidden="true"><circle cx="12" cy="12" r="1"></circle><circle cx="19" cy="12" r="1"></circle><circle cx="5" cy="12" r="1"></circle></svg></button></div></div><div class="astw:group astw:relative"><div class="astw:relative astw:size-30 astw:shrink-0 astw:overflow-hidden astw:rounded-lg astw:border astw:border-border astw:flex astw:flex-col astw:justify-between astw:bg-card astw:p-3"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-file astw:size-5 astw:text-muted-foreground astw:opacity-60" aria-hidden="true" data-testid="attachment-file-icon"><path d="M6 22a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h8a2.4 2.4 0 0 1 1.704.706l3.588 3.588A2.4 2.4 0 0 1 20 8v12a2 2 0 0 1-2 2z"></path><path d="M14 2v5a1 1 0 0 0 1 1h5"></path></svg><p class="astw:flex astw:min-w-0 astw:items-end astw:gap-0.5 astw:text-xs astw:leading-normal astw:text-foreground"><span class="astw:min-w-0 astw:flex-1 astw:line-clamp-2 astw:break-all">Aug-Sep 2025_1234-12</span><span class="astw:shrink-0">.pdf</span></p></div><div class="astw:absolute astw:top-2 astw:right-2 astw:opacity-0 astw:transition-opacity astw:group-hover:opacity-100 astw:group-focus-within:opacity-100"><button type="button" tabindex="0" aria-haspopup="menu" id="base-ui-_r_8_" data-slot="menu-trigger" aria-label="Attachment options for Aug-Sep 2025_1234-12.pdf" class="astw:inline-flex astw:size-7 astw:items-center astw:justify-center astw:rounded-md astw:border astw:border-border astw:bg-background/90 astw:text-foreground astw:shadow-xs astw:hover:bg-accent" aria-expanded="false"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-ellipsis astw:size-4" aria-hidden="true"><circle cx="12" cy="12" r="1"></circle><circle cx="19" cy="12" r="1"></circle><circle cx="5" cy="12" r="1"></circle></svg></button></div></div><button type="button" aria-label="Click to upload" data-testid="attachment-upload-tile" class="astw:flex astw:h-30 astw:w-[200px] astw:shrink-0 astw:cursor-pointer astw:flex-col astw:justify-between astw:rounded-lg astw:border-2 astw:border-dashed astw:border-border astw:bg-muted/50 astw:p-3 astw:transition-colors astw:hover:bg-muted"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-cloud-upload astw:size-5 astw:text-muted-foreground astw:opacity-60" aria-hidden="true"><path d="M12 13v8"></path><path d="M4 14.899A7 7 0 1 1 15.71 8h1.79a4.5 4.5 0 0 1 2.5 8.242"></path><path d="m8 17 4-4 4 4"></path></svg><div class="astw:min-w-0 astw:self-stretch astw:text-left"><p class="astw:text-sm astw:leading-normal astw:text-foreground">Click to upload</p></div></button></div></div></div>"`;