fix(threads): reduce dead space around root message in thread drawer

The root message section had two sources of excess whitespace:

1. hideTrack={false} forced a visible scrollbar gutter on the Scroll
   component even when the message fit within the 200px max-height and no
   scrolling was needed. Changed to hideTrack (default true = hidden) so
   the gutter only appears when actually scrollable.

2. padding: S400 0 S200 0 (16px top / 8px bottom) on the root message box
   was adding significant dead space above the message content that
   compounds with the Message component's own internal padding.
   Reduced to S200 0 S100 0 (8px top / 4px bottom).

Author: Just-Insane

src/app/features/room/ThreadDrawer.tsx

@@ -722,7 +722,7 @@ export function ThreadDrawer({ room, threadRootId, onClose, overlay }: ThreadDra
           variant="Background"
           visibility="Hover"
           direction="Vertical"
-          hideTrack={false}
+          hideTrack
           style={{
             maxHeight: '200px',
             height: 'fit-content',
@@ -733,7 +733,7 @@ export function ThreadDrawer({ room, threadRootId, onClose, overlay }: ThreadDra
             className={css.messageList}
             direction="Column"
             style={{
-              padding: `${config.space.S400} 0 ${config.space.S200} 0`,
+              padding: `${config.space.S200} 0 ${config.space.S100} 0`,
             }}
           >
             <ThreadMessage {...sharedMessageProps} mEvent={rootEvent} />