feat(sponsors): add Quansight as Silver sponsor (#210)

## Summary
Adds **Quansight** to the sponsors list at the **Silver** tier.

<img width="1105" height="1075" alt="image"
src="https://github.com/user-attachments/assets/4241fab3-a81d-4be1-a1c4-b8518d4e5980"
/>

- New data file `src/data/sponsors/quansight.md`
- New logo asset `public/sponsors/quansight.svg`

## Details
- **Logo styling:** Quansight's logo is a full-color horizontal lockup
(the wordmark is part of the SVG), so it sits on a white `logobg`
(`#ffffff`) like the other multi-color SVG sponsors (JetBrains, Veriff)
rather than the transparent-on-color approach used for Preply/Abacum.
- **Link target:** homepage (`https://quansight.com/`), consistent with
the default sponsor behavior.

## Reviewer notes
- Verified locally with `astro build` (28 pages, no errors); the Silver
tier renders the Quansight card.

Author: acabelloj

public/sponsors/quansight.svg

@@ -0,0 +1,7 @@
+---
+name: 'Quansight'
+website: 'https://quansight.com/'
+tier: 'silver'
+logobg: '#ffffff'
+logo: '/sponsors/quansight.svg'
+---