reduce positron favicon.ico image resolution (#5820)

### Summary

- Addresses #5619

### Method

- use the `magick` CLI (`brew install imagemagick` on Mac) to convert
the existing favicon.ico (1024×1024) to a lower resolution (256×256).
Thanks @petetronic for this suggestion!

```sh
magick convert favicon.ico -resize 256x256 -define icon:auto-resize=256 -quality 100 favicon2.ico
```
- replace the existing favicon.ico with the reduced-resolution
favicon.ico

### Preview

In a dev Server Web build on Mac, in Chrome:

| | Before | After |
|--------|--------|--------|
| Positron Server Web | <img width="328" alt="before"
src="https://github.com/user-attachments/assets/0dcb9280-178a-4f78-9e68-8490f3b59075"
/> | <img width="321" alt="after"
src="https://github.com/user-attachments/assets/3939d683-c32e-402c-8dce-942c16fb08f8"
/> |
| GitHub raw image preview | <img width="335" alt="image"
src="https://github.com/user-attachments/assets/ed2748f4-f041-4ef8-bc8a-9996cc98fa7e"
/>
([url](https://raw.githubusercontent.com/posit-dev/positron/81d67af667807d6f4f8f9d2a47075e7a94c5f05e/resources/server/favicon.ico))
| <img width="328" alt="image"
src="https://github.com/user-attachments/assets/76213e91-8c89-4287-b721-6d528694fa55"
/>
([url](https://raw.githubusercontent.com/posit-dev/positron/refs/heads/reduce-favicon-resolution/resources/server/favicon.ico))
|



### QA Notes

The Positron favicon should now show in dev Server Web builds as well as
in Positron on Workbench in Chromium-based browsers. All other browsers
(e.g. Firefox, Safari, etc.) should continue to display the favicon.