app: sort conversations by last user message

Conversations in the sidebar were being sorted by the timestamp of the
last message (including assistant responses). This changes the sorting
to use only user messages, matching the behavior of ChatGPT and Claude.

Fixes #12958

app/store/database.go

@@ -513,7 +513,7 @@ func (db *database) getAllChats() ([]Chat, error) {
 			WHERE role = 'user'
 			GROUP BY chat_id
 		) first_msg ON c.id = first_msg.chat_id
-		LEFT JOIN messages m ON c.id = m.chat_id
+		LEFT JOIN messages m ON c.id = m.chat_id AND m.role = 'user'
 		GROUP BY c.id, c.title, c.created_at, first_msg.content
 		ORDER BY last_updated DESC
 	`

app/ui/ui.go

@@ -1365,8 +1365,9 @@ func chatInfoFromChat(chat store.Chat) responses.ChatInfo {
 		if msg.Role == "user" && userExcerpt == "" {
 			userExcerpt = msg.Content
 		}
-		// update the updated at time
-		if msg.UpdatedAt.After(updatedAt) {
+		// update the updated at time based on user messages only
+		// this ensures conversations are sorted by last user message, not last response
+		if msg.Role == "user" && msg.UpdatedAt.After(updatedAt) {
 			updatedAt = msg.UpdatedAt
 		}
 	}