feat: add Stellar network switching UI — testnet/mainnet toggle

[sanmipaul]

Summary

The frontend has no UI control for switching between Stellar Testnet and Mainnet. Users must manually edit environment variables and redeploy to change networks, which is error-prone and blocks QA workflows.

Desired Behavior

• Network selector (Testnet / Mainnet) visible in the top navigation or settings panel
• Switching network updates the Horizon URL, contract IDs, and passphrase used by `frontend/lib/contracts.ts`
• Freighter wallet is prompted to switch to the matching network
• Active network is persisted in `localStorage` and shown as a badge in the UI
• Mainnet mode shows a prominent warning banner

Implementation Notes

• Network config should live in `frontend/lib/network.ts` with typed `NetworkConfig` interface
• Contract IDs per network stored in `frontend/lib/constants.ts`
• Use Freighter's `setNetwork()` API when switching

Acceptance Criteria

• Network toggle renders in nav
• Switching network reloads contract clients with correct IDs
• Freighter is notified of network change
• Active network badge displayed
• Mainnet warning shown
• E2E test covers toggle behavior on testnet

References

• `frontend/lib/contracts.ts`
• `frontend/lib/constants.ts` (if exists)
• Freighter API docs: `setNetwork()`

[anyijihilary-dotcom]

@anyijihilary-dotcom has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello boss, Please assign this issue to me. I checked it out and I'm confident I can solve it.

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