Syncs the resolvestroke subdataset main images on the Hub with GitHub.

- `clinical/assets/main.png`: reference microvascular image (`mvi`), x-z MIP, of SP03-Left.
- `phantom_mp/assets/main.png`: x-z B-mode sector, frame 0.
- `phantom_flow/assets/main.png`: x-z B-mode sector, frame 0, with the tube's reference `mvi` overlaid.
- `saddle/assets/main.png`: saddle B-mode sector of SP08-Left.

No data cards change.

**GitHub PR:**
- [tristan-deep/OpenH-RF#49](https://github.com/tristan-deep/OpenH-RF/pull/49) — resolvestroke: add main.png for each subdataset

Files changed: 4.

```
+ resolvestroke/clinical/assets/main.png
+ resolvestroke/phantom_flow/assets/main.png
+ resolvestroke/phantom_mp/assets/main.png
+ resolvestroke/saddle/assets/main.png
```