feat: [Preconf Phase 2] Bundler Abstraction for Preconfirmation Support

## Parent Issue
Part of #164 - Leverage EIP-7917 Based Preconfirmations for Sub-Second Transaction UX

## Overview
Create an abstraction layer in the bundler client to prepare for preconfirmation-enabled bundlers. This allows seamless integration when preconf protocols become available.

## Tasks

- [ ] Define preconfirmation types/interfaces
```javascript
// Preconfirmation receipt structure
const PreconfReceipt = {
preconfirmed: boolean,
preconfHash: string, // Hash of preconf commitment
proposerSignature: string, // Proposer's signed commitment
proposerAddress: string, // Proposer who made commitment
targetBlock: number, // Block where tx will be included
targetSlot: number, // Beacon chain slot
expiresAt: number, // Timestamp when preconf expires
userOpHash: string, // Original UserOp hash
}
```

- [ ] Create `PreconfBundlerClient` class extending `BundlerClient`
```javascript
class PreconfBundlerClient extends BundlerClient {
async getNextProposer() { /* beacon chain query */ }
async requestPreconfirmation(userOp, options) { /* preconf request */ }
async sendUserOperationWithPreconf(userOp, options) { /* main method */ }
async verifyPreconfirmation(preconfReceipt) { /* verify proposer sig */ }
}
```

- [ ] Add `sendUserOperationWithPreconf()` method
```javascript
async sendUserOperationWithPreconf(userOp, preconfOptions = {}) {
// 1. Get next proposer from beacon chain (EIP-7917 lookahead)
const nextProposer = await this.getNextProposer()

// 2. Request preconfirmation commitment
const preconfReceipt = await this.requestPreconfirmation(userOp, nextProposer)

// 3. Return immediately with preconf + async final receipt
return {
...preconfReceipt,
finalReceipt: this.waitForUserOperation(preconfReceipt.userOpHash)
}
}
```

- [ ] Add beacon chain proposer lookahead query
- Query consensus layer for `proposer_lookahead` (EIP-7917)
- Cache lookahead data (valid for epoch duration)
- Handle epoch boundaries gracefully

- [ ] Create factory function for bundler selection
```javascript
function createBundlerClient(config) {
if (config.preconfEnabled && config.preconfGateway) {
return new PreconfBundlerClient(config)
}
return new BundlerClient(config)
}
```

- [ ] Add preconf configuration to network config
```javascript
const networkConfig = {
// ... existing config
preconf: {
enabled: false, // Feature flag
gateway: null, // Preconf gateway URL
beaconRpc: null, // Beacon chain RPC for lookahead
}
}
```

- [ ] Add stub implementations (no-op for now)
- Methods should be callable but return standard bundler flow
- Easy to swap in real implementation later

## API Design

### Return Type
```javascript
{
// Preconfirmation data (available immediately)
preconfirmed: true,
preconfHash: '0x...',
proposerSignature: '0x...',
proposerAddress: '0x...',
targetBlock: 12345678,

// Final receipt (Promise, resolves after block inclusion)
finalReceipt: Promise
}
```

### Usage in TransactionSender
```javascript
const result = await bundler.sendUserOperationWithPreconf(signedUserOp)

// Immediately show preconfirmed state
setStatus('preconfirmed')
setPreconfData(result)

// Wait for final inclusion async
result.finalReceipt.then(receipt => {
setStatus('included')
setReceipt(receipt)
})
```

## Files to Create/Modify
- `frontend/src/lib/bundlerClient.js` - Add PreconfBundlerClient
- `frontend/src/lib/preconfTypes.js` - Type definitions (new file)
- `frontend/src/lib/beaconClient.js` - Beacon chain queries (new file)
- `frontend/src/config/networks.js` - Add preconf config

## Acceptance Criteria
- [ ] PreconfBundlerClient class implemented with stub methods
- [ ] Backward compatible - existing flow works unchanged
- [ ] Feature flag to enable/disable preconf path
- [ ] Clean abstraction that's easy to extend
- [ ] Unit tests for new bundler methods

## Dependencies
- Phase 1 UI components (for integration testing)

## Labels
`enhancement` `bundler` `preconfirmations`