[Feature]: 请求自定义会话分组！默认agent可以被隐藏或者排序

现在是被动的按日期分组，比如今天，7天内等。还是不方便。
希望能增加自定义分组！或者能增加标签，用标签来分类也行啊

还有default这个默认agent，删不掉改不了名字也就算了，还不能隐藏和排序靠后，也不能改路径，这就很不方便嘛！
希望至少增加排序和隐藏的功能。

The issue names no files, tests, or entry points. First locate the session-list grouping logic and the default-agent controls in the web UI, then clarify whether custom groups, tags, or both are required and how ordering and hiding should work. Done means the agreed session organization and default-agent behavior are implemented and covered by the relevant tests.

Scritto dal modello di indicizzazione a partire dal testo della issue.