fix: new message indicator, cursor desync, button overlap, avatar fallback

- #63: 'New messages' label only shows when actual new messages arrive while scrolled up; plain arrow button shown otherwise
- #64: Fix cursor desync in HighlightedTextarea by matching backdrop border width to textarea
- #65: Move floating buttons inside scroll container with sticky positioning to prevent overlap with growing textarea
- #66: Graceful fallback to Bot icon when agent avatar image fails to load

src/lib/i18n.ts

@@ -43,6 +43,7 @@ const en = {
   'chat.showPreview': 'Preview markdown',
   'chat.hidePreview': 'Hide preview',
   'chat.scrollToBottom': 'New messages',
+  'chat.scrollDown': 'Scroll to bottom',
   'chat.collapseTools': 'Collapse all tools',
   'chat.expandTools': 'Expand all tools',
   'chat.messages': 'Chat messages',
@@ -164,6 +165,7 @@ const fr: Record<keyof typeof en, string> = {
   'chat.showPreview': 'Aperçu markdown',
   'chat.hidePreview': 'Masquer l\'aperçu',
   'chat.scrollToBottom': 'Nouveaux messages',
+  'chat.scrollDown': 'Défiler en bas',
   'chat.collapseTools': 'Replier tous les outils',
   'chat.expandTools': 'Déplier tous les outils',
   'chat.messages': 'Messages du chat',