feat(config): add theme.messages_text.mention_color field

internal/config/config.toml

@@ -126,3 +126,4 @@ content_color = "white"
 emoji_color = "green"
 link_color = "blue"
 attachment_color = "yellow"
+mention_color = "blue"

internal/config/theme.go

@@ -110,5 +110,6 @@ type (
 		EmojiColor      string `toml:"emoji_color"`
 		LinkColor       string `toml:"link_color"`
 		AttachmentColor string `toml:"attachment_color"`
+		MentionColor    string `toml:"mention_color"`
 	}
 )