fix: group chat mobile UX and UI polish (#188)

* fix: group chat UI background colors and replace console.log in context-engine

- Set message list background to $bg-card to match single chat
- Set status-bar background to transparent
- Replace all console.log/warn with logger in context-engine compressor

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

* fix: group chat mobile UX improvements

- Add backdrop overlay for mobile sidebar with tap-to-close
- Auto-collapse sidebar on room select in mobile
- Move timestamp below message bubble
- Widen msg-body max-width to 85% to match single chat
- Add left padding to chat-header to avoid hamburger overlap

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

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/components/hermes/group-chat/GroupMessageList.vue

@@ -60,6 +60,7 @@ defineExpose({ scrollToBottom })
     display: flex;
     flex-direction: column;
     gap: 12px;
+    background-color: $bg-card;
 }
 
 .empty-state {