feat: add Stellar network switching UI (testnet/mainnet toggle)

Developers and advanced users need to switch between testnet and mainnet.\n\n**Action:**\n- [ ] Add network indicator in NavBar (shows current network)\n- [ ] Allow switching in developer settings\n- [ ] Warn: 'You are on testnet' banner\n- [ ] Store network preference in localStorage\n- [ ] Reload contract addresses and RPC endpoints on switch\n- [ ] In production build: disable testnet option