feat: Add Stellar Testnet/Mainnet network switcher

[dagangtj]

Description

Adds network switching functionality between Stellar Testnet and Mainnet as requested in #80.

Changes

• ✅ Added NetworkSwitcher component with dropdown in Navbar
• ✅ Updated sorosave.ts to support multiple network configurations
• ✅ Implemented localStorage persistence for network selection
• ✅ Added confirmation dialog before switching networks
• ✅ Clear cache data on network switch to prevent stale data
• ✅ Updated providers.tsx to track current network state

Implementation Details

• Network configs stored in NETWORK_CONFIGS with RPC URLs, passphrases, and contract IDs
• switchNetwork() function reinitializes SoroSaveClient with new config
• Confirmation modal warns users about cache clearing and page reload
• All sorosave_* localStorage keys (except network selection) are cleared on switch

Testing

• Network selection persists across page reloads
• Confirmation dialog appears before switching
• Cache is cleared on network change
• Client reinitializes with correct network config

Closes #80