[CmdPal][PerfMon] Use distinct up/down arrow icons for network Send/Receive (#48118)

## Summary of the Pull Request

On the Command Palette Performance Monitor extension's Network widget
page, the **Send** and **Receive** list items both used the generic
`NetworkIcon` (`\uEC05`), making them visually indistinguishable at a
glance.

This PR gives each direction its own glyph:
- Send → up arrow `\uE74A`
- Receive → down arrow `\uE74B`

The redundant `↑`/`↓` characters are removed from the Send/Receive
subtitles since the icons now carry that meaning.

Before vs after:

<img width="918" height="122" alt="image"
src="https://github.com/user-attachments/assets/4af3a2fc-d5a7-4fb5-98c6-f1889c7e80f2"
/>


## PR Checklist

- [x] **Closes:** N/A (no existing issue found)
- [x] **Communication:** I've discussed this with collaborators
- [x] **Tests:** Manually verified
- [x] **Localization:** Updated en-US resw (other locales still contain
the arrow characters and can be translated/updated by the loc pipeline)

## Detailed Description of the Pull Request / Additional comments

Files changed:
- `Icons.cs` – added `NetworkUpIcon` and `NetworkDownIcon`
- `PerformanceWidgetsPage.cs` – set `Icon` on `_networkUpItem` and
`_networkDownItem`
- `Strings/en-US/Resources.resw` – `Send ↑` → `Send`, `Receive ↓` →
`Receive`

## Validation Steps Performed

Local visual verification in Command Palette.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>