upd: implement radius setting

to keep things manageable i merged a lot of one off values into just a
handful of common sizes, so some parts of the ui will look different than
upstream even with the "Misskey" rounding mode

packages/frontend/src/components/MkChannelPreview.vue

@@ -100,7 +100,7 @@ const bannerStyle = computed(() => {
 			padding: 8px 12px;
 			font-size: 80%;
 			background: rgba(0, 0, 0, 0.7);
-			border-radius: 6px;
+			border-radius: var(--radius-sm);
 			color: #fff;
 		}
 
@@ -111,7 +111,7 @@ const bannerStyle = computed(() => {
 			left: 16px;
 			background: rgba(0, 0, 0, 0.7);
 			color: var(--warn);
-			border-radius: 6px;
+			border-radius: var(--radius-sm);
 			font-weight: bold;
 			font-size: 1em;
 			padding: 4px 7px;