feat: message grouping — hide avatar and reduce spacing for consecutive same-role messages

Consecutive messages from the same role within 2 minutes are visually
grouped: subsequent messages hide the avatar (keeping alignment) and
use tighter vertical spacing. Date separators and role changes break
groups. This matches modern chat UX patterns (Discord, Telegram).

src/index.css

@@ -52,6 +52,14 @@
   outline-offset: 2px;
 }
 
+/* Chat textarea: no focus ring (container handles visual feedback) */
+.ht-textarea:focus-visible,
+.ht-textarea:focus,
+#chat-input:focus-visible,
+#chat-input:focus {
+  outline: none;
+}
+
 /* Remove default outline for mouse/touch users */
 :focus:not(:focus-visible) {
   outline: none;