fix(wallet): surface silent save failures + stale-state cleanups (W5-1, W5-2, W6-1, W6-3)

P2 robustness batch (localized):

- W5-1 (Med, lite): persistAfterBroadcast returned false on a persistent post-send/shield
  save failure, but both callers discarded it and it never logged — completely silent. It
  now liteLogs the failure (the spent note re-derives on the next sync, so it's a
  robustness gap, not fund loss).

- W5-2 (Med, lite): the post-sync and post-rescan save results (in the detached scan
  threads) were ignored; both now liteLog on failure. LiteDiagnostics::log is mutex-guarded,
  so it's safe from those threads.

- W6-1 (Med): WalletState::clear() didn't reset mining/pool_mining, so a wallet switch could
  briefly show the previous wallet's hashrate/blocks. Now reset in clear() (the daemon
  restarts on switch, so mining genuinely stops).

- W6-3 (Low): AddressBook::load() cleared entries_ then threw on the first non-object array
  element — discarding EVERY contact. It now guards is_object() + per-entry try/catch,
  skipping and counting malformed entries.

Build-clean; ctest 1/1. Remaining P2: W6-2 (refresh-staleness badge — needs UI, overlaps
the diagnostics Foundation bundle). See docs/wallet-hardening.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/data/wallet_state.h

@@ -335,6 +335,10 @@ struct WalletState {
         transactions.clear();
         peers.clear();
         bannedPeers.clear();
+        // W6-1: reset node-level mining state too — the daemon restarts on a wallet switch (mining
+        // stops), so leaving the previous wallet's hashrate/blocks would show stale mining stats.
+        mining = MiningInfo{};
+        pool_mining = PoolMiningState{};
     }
     
     // Rebuild combined addresses list from z/t lists