Add Asset Rankings screen: top assets by holders and volume

[christabel888]

Context: Grant milestone deliverable, mobile-side. Backed by Stellar-Insightss/backend#39.

Where:

• New src/screens/main/AssetRankingsScreen.tsx

What to do:

• Build a screen listing top assets by holders and volume.

Suggested approach:

1. Backed by Stellar-Insightss/backend#39. Support both sort modes (holders, volume) rather than building two separate screens.

Watch out for:

• Coordinate nav placement with Restructure MainNavigator.tsx tabs: Overview, Assets, Wallets, Soroban, Rankings, Explorer #36's Rankings group.

Definition of done:

• Screen exists, reachable from the Rankings tab (Restructure MainNavigator.tsx tabs: Overview, Assets, Wallets, Soroban, Rankings, Explorer #36), showing real data