Multi-account support for transaction submission

## Summary

Support submitting blobs from multiple funded accounts to avoid nonce contention when writing to a single namespace at high throughput.

## Problem

When a single account submits transactions concurrently, nonce (sequence number) conflicts cause failures:
- Tx A gets nonce 5, Tx B gets nonce 5 → one fails with "account sequence mismatch"
- Serial submission avoids this but kills throughput
- Mempool ordering is not guaranteed, so even sequential nonce assignment can race

## Design

### Account pool
- Configure multiple funded accounts (key/mnemonic per account)
- Round-robin or least-recently-used assignment when a submission is requested
- Each account manages its own nonce sequence independently

### Nonce management
- Track pending nonce per account locally (don't rely solely on on-chain query)
- Increment optimistically on broadcast, roll back on rejection
- Handle sequence mismatch errors by re-querying and retrying from the correct nonce

### Configuration
```toml
[submission]
enabled = true

[[submission.accounts]]
name = "submitter-1"
key_file = "./keys/submitter-1.json"

[[submission.accounts]]
name = "submitter-2"
key_file = "./keys/submitter-2.json"

[submission.pool]
strategy = "round-robin" # or "least-pending"
max_pending_per_account = 4
```

### Safety
- Monitor account balances, alert when low
- Cap max pending transactions per account to bound risk
- Graceful degradation: if an account is unhealthy (repeated failures, low balance), remove from rotation

## Dependencies

- Requires #4 (custom transaction submission client) — nonce management is tightly coupled with how we build and broadcast transactions

## References

- Cosmos SDK sequence/nonce model: `auth` module `BaseAccount.Sequence`

Start by reviewing the custom transaction submission client in dependency issue #4 and the Cosmos SDK auth module's BaseAccount.Sequence reference. Done means supporting configured funded accounts with independent nonce handling, account selection, safety limits, and graceful removal of unhealthy accounts as described in this issue.

Vom Indexierungsmodell aus dem Issue-Text verfasst.