Add native share-sheet (Twitter/X) action on TopMoversCard

[christabel888]

Context: Pivot doc explicitly calls out shareability as the point of this feature, same as the frontend issue.

Where:

• src/components/TopMoversCard.tsx

What to do:

• Add a native share action (React Native Share API) with an X/Twitter-friendly pre-filled message.

Suggested approach:

1. Use React Native's Share API with a pre-filled message; test the actual share sheet on both iOS and Android since their share UIs differ.

Watch out for:

• Don't assume Web-Share-API-style fallback logic from the frontend issue applies here -- mobile has its own native share mechanism, use it directly.

Definition of done:

• Share sheet opens with pre-filled text on tap