bug: NewBackend() panics instead of returning error on config/client failures

[Aboudjem]

Bug Description

In rpc/backend/backend.go, NewBackend() uses panic() for two failure conditions:

appConf, err := config.GetConfig(ctx.Viper)
if err != nil {
    panic(err)  // Line ~210
}

rpcClient, ok := clientCtx.Client.(tmrpcclient.SignClient)
if !ok {
    panic(fmt.Sprintf("invalid rpc client..."))  // Line ~215
}

Impact

• Any operator misconfiguration in app.toml causes an unrecoverable crash with an unhelpful panic trace
• A wrong client type produces a panic instead of a descriptive error
• All 5 callers in rpc/apis.go have no way to handle these failures gracefully

Proposed Fix

Change NewBackend signature to return (*Backend, error) and replace panics with error returns. Update all 5 callers in rpc/apis.go.

[linear]

EVM-441

[swift1337]

Closing for being a low-effort AI submission.