feat(comments): denser comments panel (tighter spacing, 14px body)

Reduce vertical/horizontal air in the Comments side panel and shrink
body/author/selection font sizes for more comments per screen, scoped to
features/comment/ so the shared aside (TOC/Details tabs) is untouched.

- comment-list-item: author name + selection quote sm->xs, avatar gap xs,
  outer pb 6
- comment-list-with-tabs: Paper p/mb sm->xs, reply Divider my 4->2
- comment.module.css: body ProseMirror font-size sm (16->14px), line-height
  1.4, margin-top 10->4; .textSelection padding 8->6, margin-top 4->2

Implements docs/backlog/comments-panel-density.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/comment/components/comment-list-item.tsx

@@ -116,8 +116,8 @@ function CommentListItem({
   }
 
   return (
-    <Box ref={ref} pb="xs">
-      <Group>
+    <Box ref={ref} pb={6}>
+      <Group gap="xs">
         <CustomAvatar
           size="sm"
           avatarUrl={comment.creator.avatarUrl}
@@ -126,7 +126,7 @@ function CommentListItem({
 
         <div style={{ flex: 1 }}>
           <Group justify="space-between" wrap="nowrap">
-            <Text size="sm" fw={500} lineClamp={1}>
+            <Text size="xs" fw={500} lineClamp={1}>
               {comment.creator.name}
             </Text>
 
@@ -177,7 +177,7 @@ function CommentListItem({
             tabIndex={0}
             aria-label={t("Jump to comment selection")}
           >
-            <Text size="sm">{comment?.selection}</Text>
+            <Text size="xs">{comment?.selection}</Text>
           </Box>
         )}
 

apps/client/src/features/comment/components/comment-list-with-tabs.tsx

@@ -121,8 +121,8 @@ function CommentListWithTabs() {
       <Paper
         shadow="sm"
         radius="md"
-        p="sm"
-        mb="sm"
+        p="xs"
+        mb="xs"
         withBorder
         key={comment.id}
         data-comment-id={comment.id}
@@ -145,7 +145,7 @@ function CommentListWithTabs() {
 
         {!comment.resolvedAt && canComment && (
           <>
-            <Divider my={4} />
+            <Divider my={2} />
             <CommentEditorWithActions
               commentId={comment.id}
               onSave={handleAddReply}

apps/client/src/features/comment/components/comment.module.css

@@ -7,9 +7,9 @@
 }
 
 .textSelection {
-    margin-top: 4px;
+    margin-top: 2px;
     border-left: 2px solid var(--mantine-color-gray-6);
-    padding: 8px;
+    padding: 6px;
     background: var(--mantine-color-gray-light);
     cursor: pointer;
     overflow-wrap: break-word;
@@ -39,7 +39,11 @@
         word-break: break-word;
         padding-left: 6px;
         padding-right: 6px;
-        margin-top: 10px;
+        /* Denser comments: shrink body text from the global 16px ProseMirror size
+           to 14px and tighten the rhythm vs. the comment header. */
+        font-size: var(--mantine-font-size-sm);
+        line-height: 1.4;
+        margin-top: 4px;
         margin-bottom: 2px;
     }