enhance notification

### Feature Description

I think these features will really make notificaiton better:

- allow user to customize the number of line which notification display
- allow user to expend the notification message, rather than go to notificaiton center than expend it

### Use Case

sometimes the message part of notificaiton will be more than two lines, so I think these new feature will make it better

### Compositor(s)

niri

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

Start by locating the notification UI and notification center in DankMaterialShell, then inspect how notification message lines and expansion are currently handled. Clarify the intended line-count setting, expansion interaction, and compositor-specific behavior before implementation; done should cover both requested behaviors with tests or reproducible checks.

Written by the indexing model from the issue text.