feat: add gas estimation display before transaction submission

[sanmipaul]

Summary

Before users sign any Soroban transaction, the Astera frontend should simulate the transaction and display the estimated gas cost (in XLM). Currently, users have no way to know the cost until the Freighter wallet shows the fee — at which point they've already invested time in filling out forms.

Desired Behavior

On any action form (create invoice, deposit, fund invoice, repay), after the user has filled in all fields:

1. Frontend calls `sorobanRpc.simulateTransaction()` to get the compute units
2. Estimated fee shown below the submit button:
   ```
   Estimated network fee: ~0.00123 XLM
   [Sign & Submit]
   ```
3. If simulation fails (e.g., insufficient collateral), show the error inline before the user signs

Implementation

```ts
// frontend/hooks/useTransactionSimulation.ts
export function useTransactionSimulation(operation: xdr.Operation) {
return useQuery({
queryKey: ['simulate', operation],
queryFn: async () => {
const server = new SorobanRpc.Server(horizonUrl);
const result = await server.simulateTransaction(buildTx(operation));
if (SorobanRpc.Api.isSimulationError(result)) throw new Error(result.error);
return result;
},
enabled: !!operation,
});
}
```

Acceptance Criteria

• Transaction simulation runs when user fills in all required fields
• Estimated fee displayed in XLM and USD equivalent
• Simulation errors shown inline (e.g., "Insufficient collateral")
• Loading spinner shown during simulation
• Submit button disabled until simulation completes (or shows warning if skipped)
• Unit test: simulation error → inline error message shown

References

• `frontend/components/` — form components
• Soroban RPC: `simulateTransaction()`