[Design quality]: Update header styling for Portfolio View

### Describe the bug

The header shadow makes the experience feel heavy. Updating the styling creates a more modern, polished look.

### Expected behavior

For `multichain-app-header`
- `multichain-app-header-shadow` is removed
- `border-bottom: 1px solid var(--color-border-muted)` is applied

### Screenshots/Recordings

[Figma link](https://www.figma.com/design/aMYisczaJyEsYl1TYdcPUL/Portfolio-View?node-id=5136-39406&t=ScwAp5cXkdFelVda-4)

### Steps to reproduce

Open MetaMask

### Error messages or log output

_No response_

### Detection stage

In production (default)

### Version

12.2.4

### Build type

None

### Browser

Brave

### Operating system

MacOS

### Hardware wallet

_No response_

### Additional context

Ping @amandaye0h for context and final review

### Severity

_No response_