feat(focus-rings): CommentThread selection highlight → dotted outline

ring-2 ring-accent (box-shadow) replaced with outline-2 outline-dotted
outline-accent — visually distinct from the focus ring (solid, navy/mint),
making selection state and keyboard focus clearly different

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/CommentThread.svelte

@@ -312,7 +312,7 @@ onMount(async () => {
 			<div
 				data-comment-id={thread.id}
 				class={highlightedCommentId === thread.id
-					? 'rounded ring-2 ring-accent ring-offset-1 transition-shadow'
+					? 'rounded outline-2 outline-offset-1 outline-accent transition-shadow outline-dotted'
 					: ''}
 			>
 				{@render commentEntry(thread, thread.id, thread.replies.length === 0)}
@@ -323,7 +323,7 @@ onMount(async () => {
 				<div
 					data-comment-id={reply.id}
 					class="mt-3 ml-6 border-l-2 border-line pl-4 {highlightedCommentId === reply.id
-						? 'rounded ring-2 ring-accent ring-offset-1 transition-shadow'
+						? 'rounded outline-2 outline-offset-1 outline-accent transition-shadow outline-dotted'
 						: ''}"
 				>
 					{@render commentEntry(reply, thread.id, ri === thread.replies.length - 1)}