upsert_account_storage uses two separate Dexie transactions partial write hazard

## Bug

`upsert_account_storage` in `crates/idxdb-store/src/account/utils.rs` calls `idxdb_upsert_account_storage` and `idxdb_upsert_storage_map_entries` as two separate Dexie transactions. If the second call fails after the first succeeds, storage slot headers are persisted without their corresponding map entries, leaving the account in an inconsistent state.

## Impact

- Storage slot headers reference map roots that have no entries in the database
- Storage reconstruction fails validation
- Inconsistency persists until full re-sync

## Root cause

Each helper creates its own Dexie transaction. No outer transaction wraps them.

Same class as #190 but a different code path.

The bug is in `crates/idxdb-store/src/account/utils.rs` in the `upsert_account_storage` function. Examine the two separate calls to `idxdb_upsert_account_storage` and `idxdb_upsert_storage_map_entries`. Understand how Dexie transactions work in this codebase. The fix likely involves wrapping both operations in a single transaction. Check for existing patterns or tests related to transaction handling, and run any tests for storage consistency.

Written by the indexing model from the issue text.