fix(ui): round the album grid hover overlay in the Nautiline theme (#6115)

The theme rounded the cover image directly and set a border radius on
albumContainer, which has no background or clipping, so it rounded
nothing. The hover overlay is a sibling of the image inside the same
link, so it kept square corners that poked out over the rounded cover.

Move the radius to that link and clip it, so both the image and the
overlay follow the same rounded box. This also covers the mobile bar,
which is always visible.

Fixes #6110

ui/src/themes/nautiline.js

@@ -598,11 +598,9 @@ const NautilineTheme = {
       },
     },
     NDAlbumGridView: {
-      albumContainer: {
+      link: {
         borderRadius: radii.md,
-        '& img': {
-          borderRadius: radii.md,
-        },
+        overflow: 'hidden',
       },
       albumTitle: {
         fontWeight: 600,

ui/src/themes/theme.test.js

@@ -12,3 +12,25 @@ describe('NDPlaylistDetails styles', () => {
     },
   )
 })
+
+describe('NDAlbumGridView styles', () => {
+  const themeEntries = Object.entries(themes)
+
+  // The hover overlay is a sibling of the image, so it keeps square corners.
+  it.each(themeEntries)(
+    '%s should not round the grid cover image on its own',
+    (themeName, theme) => {
+      const container = theme.overrides?.NDAlbumGridView?.albumContainer
+      expect(container?.['& img']?.borderRadius).toBeUndefined()
+    },
+  )
+
+  it.each(themeEntries)(
+    '%s should clip the grid cover link when it is rounded',
+    (themeName, theme) => {
+      const link = theme.overrides?.NDAlbumGridView?.link
+      if (!link?.borderRadius) return
+      expect(link.overflow).toBe('hidden')
+    },
+  )
+})