Blockchain reorg safety

`Client::sync_state` currently hardcodes `SyncTarget::CommittedChainTip`, even though the RPC layer and node already support `CommittedChainTip`, `ProvenChainTip`, and `BlockNumber` via the existing `SyncTarget` enum.
We could consider exposing that as an optional parameter on `Client::sync_state`, thread it through `StateSync::sync_state` into `sync_chain_mmr`, reexport the enum, and maybe mirror the parameter in the CLI and `WebClient` bindings. I don

This does not add reorg safety, and maybe this prompts for discussion/features around that: The client still has no detection or rollback for network-level chain rewrites. The existing account rollback logic only handles locally stale pending transactions. A user choosing `ProvenChainTip` gets a more conservative view, but still fully trusts the node, so `proven` is not a stronger trust guarantee.