Allow custom RPC configuration per chain

[gabitoesmiapodo]

Split from #61

User story / Problem statement

Currently, the dApp uses preconfigured RPC endpoints. Developers and testers need the ability to connect to custom RPCs (e.g., Tenderly forks) to test against specific chain states or forked environments.

Expected outcome

Users can configure custom RPC URLs per chain through a settings section in the dApp, overriding the default endpoints.

Acceptance criteria

• A configuration UI allows setting custom RPC URLs per chain
• Custom RPCs override the default transport for the specified chain
• Configuration persists across sessions (e.g., localStorage)
• Invalid RPC URLs are validated and errors are surfaced to the user