Add border to the bottom of the header in Mobile

### Describe the bug

Currently, we don't have a border to distinguish the header from the rest of the content. The lack of delineation makes the home page feel unstructured. which can make the layout feel unstructured. Adding a subtle divider, creates more distinction between the content.

### Expected behavior

- New header has `border-bottom: 1px solid color-border-muted`
- Padding matches the designs

### Screenshots/Recordings

### Steps to reproduce

Open MetaMask

### Error messages or log output

```shell

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### Detection stage

In production (default)

### Version

Latest version

### Build type

None

### Device

Android

### Operating system

Android

### Additional context

_No response_

### Severity

_No response_