feat: offline mode

[dorianvp]

No description provided.

[zancas]

This mnemonic is used elsewhere.. the comment seems to imply that it's used here EXCLUSIVELY.

[zancas]

1. Make the Account trait methods not panic on reachable paths.

My concern is that we may have turned a compile-time gap into a runtime panic: if any zcash_client_backend path we now reach calls one of these accessors, it aborts.

We MUST guarantee at compile time that the accessor calls can't cause a panic (for example... even if there's a malicious ZCB).

Can we write an adversarial test that causes a panic?

2. Avoid the shared temp-file race between flush() and save_task.

Both call write_to_path, which always writes to the same wallet_path.with_extension("…tmp"). The wallet().write() lock serializes serialization but not the subsequent file write, so a consumer calling flush() while the background save_task is running can have two writers truncate/interleave the same .tmp before the rename.

Please add an adversarial test that causes interleaving and is fixed by uniquifying the temp filename.

3. Don't flatten the offline error to a debug string in do_info.

The offline branch returns format!("{e:?}"), which collapses to "Offline" and loses the helpful Display message ("Offline: no indexer configured. Call set_indexer_uri() to connect.").

Make do_info return a structured type, so we can leverage Display properly.

4. Add an offline → online transition test.

The current tests cover the offline error paths thoroughly but not the transition that's the whole point of the feature:

construct offline, call set_indexer_uri(), then confirm network ops no longer return Offline.

Even a no-network assertion that indexer_uri().is_some() after set_indexer_uri would lock in the happy path.

[zancas]

See my comment.

[zancas]

Add tests proving that neither pepper-sync, nor any other network-requiring functionality, CAN be exercised in offline-mode.

These guarantees MUST be fully auditable at compile-time.

I.E. All paths that exercise network-dependent logic must be conditionally INaccessible in offline-mode, and that condition MUST be enforced at a Single Point.

[zancas]

Consider an offline-only feature-gate that compiles only offline functionality.

[dorianvp]

Another TODO: feature-gate online capabilities AND make it the default!